Boris Johnson has confirmed the government will go ahead with trials of so-called Covid passports in England as lockdown restrictions begin to ease.
The trials will be part of the government's review into Covid certificates. When the review carried out by Cabinet Office minister Michael Gove is complete, the proposal will be put to a vote in Parliament.
Some MPs are opposing the measure, saying certificates would be discriminatory to those who do not want to be vaccinated, while others say they offer a way to reopen venues such as theatres and nightclubs.
Here's what we know so far:
How will it work?
A Covid certification scheme for large-scale events will begin in England in April that will test how large events can start to take place without the need for social distancing.
Certification will be given to individuals that have either received the vaccine, tested negative recently or have "natural immunity" having had the virus in the previous six months.
It is not clear whether the 'passport' will be issued on an app or as a physical document but the NHS is said to be currently working on ways of providing the information through "digital and non-digital routes".
When and where is the scheme being trialled?
Specific events have been named as part of the Events Research Programme (ERP) which could include certification in April and May.
All are large-scale events varying in size from an audience of 300 to crowds of up to 21,000.
They government said the following events are included in the scheme:
FA Cup Semi Final, Wembley Stadium (18 April)
World Snooker Championship, Sheffield Crucible Theatre (17 April - 3 May)
Luna Cinema, Liverpool (23-25 April)
Mass participation run, Hatfield House, Hatfield (24-25 April)
Carabao Cup Final (25 April)
ACC Business Event, Liverpool (28 April)
Circus Nightclub, Liverpool (30 April - 1 May)
FA Cup Final, Wembley Stadium (15 May)
It will mean spectators and revellers at some of these events will have to provide certification or be tested before and after the event.
However, Liverpool council said that a 'Covid passport' will not be trialled at events in the city and instead will be using social distancing, ventilation and test-on-entry protocols.
Have there been any issues?
An event at the Hot Water Comedy Club in Liverpool had been scheduled for 16 April, but the venue has since pulled out of the scheme following "confusing messaging" from the government.
The club had signed up for the event to "prove that venues and events like ours could be safely reopened and take place".
But the venue said after the government released a press statement around 'vaccine passports' it was unclear which trial the comedy club was in and many drew the wrong conclusions.
The club said: "For some reason, all trials had been bundled together under the same press release and it seemed that we were part of the vaccine passport programme."
It added it was then subject to a "hate campaign", which has "significantly damaged our business and brand".
I'm attending one of the events, what will I have to do?
People attending the trials will have to adhere to an agreed code of behaviour when they purchase a ticket and to take a Covid test both before and after the event.
They will be required to follow existing Government guidance, including wearing face coverings, and to provide contact details of everyone in their group for NHS Test and Trace.
Will Covid passports end up being used in other places?
Ministers believe the scheme will be most useful in managing the risks where there are large numbers of people in close proximity, such as music festivals, sporting matches and nightclubs.
It will not apply initially to businesses which are set to reopen over the coming weeks such as pubs, restaurants and non-essential retail. But the ongoing government review suggest they could be used in pubs and restaurants to reduce social distancing restrictions.
Settings where certification will not be required include venues providing essential services, including supermarkets, public transport and GP surgeries.
Vaccine passports to facilitate international travel are very likely to be required in the future, the PM said on April 6.
He said several other countries are also looking at “the role of vaccination passports for overseas travel”, which is “going to be a fact of life, probably”.
When will the Covid certificates be used?
Mr Johnson confirmed certificates would not be used in the next two steps out of lockdown, on April 12 and May 17.
The government review into the idea is expected to be completed in the summer. It will then need to be voted through by Parliament, which could prove tricky due to some opposition.
A government document published on April 5 suggested they will eventually be in use. "Even without government intervention, Covid-status certification is likely to become a feature of our lives until the threat from the pandemic recedes," the document said.
Will there be any exemptions?
Officials are working with clinical and ethical experts to ensure there are “appropriate exemptions” for people who are advised to not have the vaccine and for whom repeat testing would be difficult.
Is there any opposition to the scheme?
Although certification is likely to include testing and immunity as well as vaccination, the move has been opposed by a cross-party coalition of more than 70 MPs - including up to 41 Tories. Labour is also sceptical about the measure.
Senior Tories such as David Davis and Sir Graham Brady, chairman of the powerful backbench 1922 Committee, have indicated they would vote against the plan.
Tory MP Steve Baker, deputy leader of the Covid Recovery Group, labelled the proposals "entirely un-British" as he demanded Health Secretary Matt Hancock explain in the House of Commons on April 6 why Covid certification would be necessary.
He said: "The last thing we should do is allow Covid to have the victory of changing our country forever into the miserable dystopia of Checkpoint Britain."
Senior Tory Mark Harper, chairman of the lockdown-sceptic Covid Recovery Group, warned Covid status certification "will lead to a two-tier Britain".
Former Labour leader Jeremy Corbyn warned it was "divisive and discriminatory" and it also looks like the current leadership will be called for its MPs to reject any motion.
Sir Keir Starmer said it would be against the "British instinct" to use certification.
Shadow health secretary Jonathan Ashworth told BBC Breakfast that Labour was "very sceptical" about the idea and wants more details about how they would work.. He said while it "makes sense" to ask people to take a test before going to events such as football games, asking people for a vaccination passport is "discriminatory".
Liberal Democrats leader Ed Davey is also opposed to the proposal.
Cabinet Office minister Michael Gove, who has led the task force responsible for drawing up the plans, acknowledged it raised “a host of practical and ethical questions” which needed to be resolved before there could be any wider rollout.
However he said that it was essential the government took the lead, otherwise venues and other businesses would simply begin setting up their own certification schemes.
“This is not about a vaccine passport, this is about looking at ways of proving that you are Covid secure, whether you have had a test or had the vaccine" he told BBC News.
“Clearly, no decisions have been made on that, because we have to weigh up different factors, the ethical considerations and so on, but it may be a way of ensuring we can get more people back doing the things they love.”
Dr Mike Ryan, executive director of the World Health Organisation’s health emergencies programme, warned that vaccine passports for travel could isolate countries struggling with vaccine acess.
What has Boris Johnson said about the trials?
The prime minister said there were “ethical and practical issues” with vaccine passports but some test events will be launched in the next few weeks.
He told a Downing Street briefing on April 5: “I want to stress there are complicated ethical and practical issues as I think I said last time raised by the idea of Covid status certification using vaccination alone.
“Many people will be for one reason or another unable to get a vaccine, for medical reasons for instance, or perhaps because they’re pregnant.
“So you have to be very careful how you handle this and don’t start a system that is discriminatory.
“But obviously we are looking at it – we want to be going ahead in the next few weeks with some test events, some pilot events. Big events, getting 20,000 people into Wembley on May 15, that kind of thing.
“Getting people back into theatre, that will unquestionably involve testing to allow the audience really to participate in the numbers that people want.”
He added the Government was “some way off finalising any plans” for so-called vaccine passports.
“The principle of requiring some people to have a certificate to prove they are not passing on the disease, like surgeons who have to prove they are vaccinated against hep B or whatever, that can be a sensible one,” he told a press conference.
“But I want to stress that we are some way off finalising any plans for Covid certification in the UK.