Video report by ITV News Health Correspondent Emily Morgan
Coronavirus could lead to a fifth of workers being absent, cause the police to drop low-priority cases and force the NHS to delay non-urgent care, according to the Government's battle plan to tackle Covid-19.
Also on Tuesday, the World Health Organization revised the mortality rate for Covid-19 up to 3.4% from 1%.
The Government's "coronavirus action plan" reveals several emergency measures could be enacted if the virus becomes a "severe prolonged pandemic".
"In a stretching scenario", the 27-page document says, "it is possible that up to one fifth of employees may be absent from work during peak weeks".
In the event of the "outbreak worsening" pressures on society and services may become "significant and clearly noticeable".
The plan comes as NHS England have upgraded coronavirus to a level four incident - the highest category.
As of Tuesday morning, the number of coronavirus cases in the UK had risen to 51, Health Secretary Matt Hancock said in a House of Commons address.
The 12 new cases reported on Tuesday included two people in Bury, another from Bolton, with the other cases in London, Hampshire, Northamptonshire, Wirral, Humberside and Kent.
Chief medical officer Prof Chris Whitty said all cases were being investigated and contact tracing has begun.
He added: "Eight patients had recently travelled from Italy, one from Germany, one from Singapore, one from Japan and one from Iran. "
Is the UK running business as usual? ITV News' John Ray looks at the facts
In a press conference on Tuesday, Prime Minister Boris Johnson said it is "highly likely" the UK will see a growing number of coronavirus cases, but stressed that "for the vast majority of people in this country we should be going about our business as usual".
England's chief medical officer Professor Chris Whitty said "probably around 1% of people who get this virus might end up dying", but the death rate varies according to age group.
Mr Whitty said "there is no evidence that children are particularly badly affected by this virus", but added there is a "significant ramp up" in the death rate of those infected who are over 80 years old.
He stressed, however that the "great majority" survive coronavirus, even among the elderly.
Measures for the police
If large numbers of police become infected, available officers may be instructed to drop low priority cases, the government action plan states.
To ensure "critical functions" are fulfilled, officers may only "concentrate on responding to serious crimes and maintaining public order".
In order to contain the spread of the virus, police and medical professionals may be given the power to detain people suspected of being infected.
The armed forces will be on standby with "well practiced arrangements" to assist the police and other civil authorities should they face pressures and request assistance.
"The army is of course always ready to back-fill as and when, but that is under the reasonable worst case scenario," the prime minister said.
If NHS staff "start to become affected" then retirees and those on leave may be called "back to duty" to assist with care.
Non-urgent care could also be delayed to prioritise triage service delivery.
The government wants to delay a major outbreak till the warmer months, when pressure on the NHS from seasonal illnesses like flu should have reduced.
The document details how the health and social care systems have "plans in place" to ensure people receive essential care and support but "sometimes this might mean that other services are reduced temporarily".
There may also be a need to draw on the UK's existing medical stockpiles as the "requirement for medicines and other clinical counter-measures may start to increase".
A "distribution strategy" for the UK's stockpiles of key medicines and equipment such as protective clothing, which will cover the NHS, and could extend to social care.
"There could be an increase in deaths arising from the outbreak, particularly amongst vulnerable and elderly groups," the document said, and advice will be provided to local authorities to help deal with this "challenge".
Labour leader Jeremy Corbyn urged the government to provide extra funding for the NHS.
"The NHS needs support in order to cope with what could become a very serious problem throughout our health service," he said.
"I hope the government will recognise the need to put the funding in."
The government is encouraging an increase in home working to avoid a spread of the virus and says people experiencing symptoms should self-isolate.
A growing concern among the workforce is that many people forced to self-isolate could be off work without pay, as ITV News Political Editor Robert Peston explains.
When asked about the issue by ITV News Political Editor Robert Peston, the prime minister did not offer much reassurance to those on zero hour contracts who, as things stand, would not be paid while off work.
"On the issue of statutory sick pay and the risks that the workforce could conceivably run in that reasonable worst case scenario, we're going to keep all options under review but we are well aware of the issue," Mr Johnson said.
Jeremy Corbyn said people on zero hour contracts will not, as things stand, be paid if they're forced to self-isolate.
"The government needs to be clear that it's going to extend sick pay arrangements to all workers," he said.
"Otherwise people through economic circumstances, will end up not undertaking the necessary self-isolation which helps to keep us all safe."
Schools and large gatherings
Measures aimed at delaying the spread of the virus could include school closures and "reducing the number of large-scale gatherings" such as football matches.
With some school closures already in force, the Department for Education is setting up a helpline to manage increasing queries about coronavirus and schools.
Outlining measures to delay the spread of the virus, the Government document says: "Action that would be considered could include population distancing strategies (such as school closures, encouraging greater home working, reducing the number of large scale gatherings) to slow the spread of disease throughout the population, whilst ensuring the country's ability to continue to run as normally as possible.
"The UK government's education departments' planning assumptions include the possibility of having to close educational settings in order to reduce the spread of infection.
"We would consider such measures in order to protect vulnerable individuals with underlying illnesses, and thus at greater risk of becoming seriously affected by the disease.
"The effectiveness of these actions will need to be balanced against their impact on society."
Banning travel to certain countries because of the coronavirus will make "no difference" if the outbreak reaches a global epidemic, according to the chief scientific adviser to the Government.
Sir Patrick Vallance told reporters that if the Covid-19 virus spreads in the UK, it "doesn't really make more sense to say 'you're at more risk somewhere else than you are here'."
He went on: "Once the epidemic is everywhere, then actually restricting travel makes no difference at all.
"At the moment we are certainly not recommending any change to behaviours in relation to that."
But England's chief medical officer Professor Chris Whitty said it would be "more problematic" for UK tourists visiting countries "with a very weak health service at the peak of their epidemic".
This is "particularly an issue" for people who are older or have pre-existing health conditions, he warned.
He added: "In those situations, they just might want to think through the practicalities of being in a place which has a health service less strong than the NHS."
The latest Foreign and Commonwealth Office advice on Covid-19 only recommends against all but essential travel to mainland China, and parts of South Korea and northern Italy.
The document reiterates how the best way for the public to help contain the virus is to regularly wash hands for at least 20 seconds.
It also asks the public not to spread misinformation on the virus and advises people to ensure family vaccines are up to date.
Checking on family, friends and neighbours is important and people should check Foreign Office advice before travelling abroad and be understanding of the pressures the health service is under. Certain events are issuing advise to people attending, as ITV News Consumer Editor Chris Choi explains.
The advice for managing Covid-19 for most people, the government says, "will be self-isolation at home and simple over the counter medicines."
People are also warned that the strain of coronavirus is new and people have a lack of immunity to it, meaning "Covid-19 has the potential to spread extensively".
On Tuesday morning London Mayor Sadiq Khan said he was avoiding shaking people's hands, in order to avoid infection, but Mr Johnson said he is and will continue to shake hands.
Mr Khan said he is not shaking hands because he is unable to wash his hands as regularly as he should, but Mr Johnson said as long as hands are washed before, then hand shaking is fine.
He said: "I am shaking hands, I was at a hospital the other night where I think there were coronavirus patients and I was shaking hands with everybody, you will be pleased to know, and I continue to shake hands.
"People must make up their own minds but I think the scientific evidence is... our judgment is that washing your hands is the crucial thing."
Health Secretary Matt Hancock told BBC Breakfast he understood why people may not want to shake hands, but added: "The scientific advice is that the impact of shaking hands is negligible and what really matters is that you wash your hands more often."
Phases of response
The Government's response is in four stages: containing the outbreak, delaying its spread and mitigating the impact of the disease once it becomes established, and alongside that, a research programme is aimed at improving diagnostics and treatment for the disease.
The government says it is currently in the "contain and research phase".
It will escalate its response upon advice from the UK's chief medical officers, who will consider the "degree of sustained transmission" and failed measures in other countries.
When the response moves to phases "delay, through to mitigate", there will be more regular Cobra meetings and there will be more government communication with Parliament and the media.
New laws could also be implemented to help systems and services "work more effectively in tackling the outbreak".