Video report by ITV News Political Correspondent Carl Dinnen
Included in a host of changes to England's Covid-19 lockdown announced on Friday, the PM said from 1 August the government will "update its advice on going to work".
The PM said employers will have "more discretion" to make decisions on how staff can "work safely" - with bosses deciding whether that is from home or in the workplace.
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Quizzed on the changes to work from home advice, Mr Johnson told ITV News: "Our view is that it is safe provided that employers have taken the steps that they need to take" to ensure a workplace is "Covid secure," Mr Johnson said.
He added: "Whether people should go into work, whether they need to go into work, is not something that the government can decide.
"It is up to employers to decide, with their employees, whether they think the time has come for them to be more productive, in a place of work for a greater portion of the week, than staying at home."
He added there was sometimes "no substitute" for face-to-face working.
The change in policy has been met with concern from some, however, with the TUC trade union federation accusing the PM of "passing the buck on this big decision to employers".
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The Confederation of British Industry (CBI) welcomed the move but warned "changing behaviour and confidence will take time".
CBI deputy director general Josh Hardie said: "Some jobs cannot be done from home. Some homes are not good places to work from."
Mr Hardie continued: "Low demand and footfall on many high streets are leaving firms struggling, driving up unemployment".
London mayor Sadiq Khan also raised concerns over the move, warning Tubes in the capital cannot be allowed to become "packed" again.
He told the PA news agency: "It’s really important if the government is going to be asking people to return to work they do so safely.
"I’d ask employers to make sure start and finish times are staggered to avoid the rush hour. It's really important that we don’t have a return to work that leads to a second wave."
In further easing of restrictions, most remaining leisure centres will be allowed to reopen in England from the August 1 the PM announced - including "bowling, skating rings, and casinos".
Indoor performances with live audiences will also be allowed to restart pending pilots with socially distanced guests.
While trials will begin of larger gatherings in places like sports stadiums "with a view to a wider reopening in the autumn," Mr Johnson announced.
Mr Johnson also immediately scrapped the advice to avoid public transport in England.
"We will enable close contact services, beauticians to resume," said the PM as well as wedding receptions for up to 30 people.
Mr Johnson added: "It is my strong and sincere hope that we will be able to review the outstanding restrictions and allow a more significant return to normality from November, at the earliest, possibly in time for Christmas."
Asked how optimistic he was that families could spend Christmas together, Mr Johnson said: "It's very important that we hope for the best, that’s what I’m trying to set out today."
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The PM also announced new powers for local authorities to impose local lockdown restrictions, allowing for "targeted local action" to combat outbreaks.
From Saturday a local authority will be able to close specific premises, shut outdoor spaces and cancel events to try and control any coronavirus outbreaks.
Announcing the measures, Mr Johnson said: "It has to be right that we take local action to contain local outbreaks".
The PM also announced an extra £3 billion in funding to prepare for a possible second wave of the coronavirus pandemic and "plan for the worst".
Mr Johnson warned Covid-19 could become "more virulent" in winter.
Winter always places a great strain on the health service, but there are concerns a resurgence of the virus during the annual flu season could be crippling.
The research said capacity for 350,000 tests per day will be needed to test individuals as they show symptoms of either Covid-19 or flu.
The latest government figures say capacity stands at nearly 338,000.
Mr Johnson has now committed to increase antigen testing capacity to 500,000 per day - or 3.5 million per week - by the end of October.
The PM said: "As we approach winter we will need to go further, not least as many more people will show Covid-like symptoms as a result of seasonal illnesses and therefore require a test."
Alongside the PM at Friday's briefing, Baroness Dido Harding set out how Test and Trace would improve in the lead-up to the winter months.
She admitted: "There is lots more for NHS Test and Trace to do," adding:
"Making sure that NHS Test and Trace is ready for winter is the job for me and my team to do over the summer but NHS Test and Trace depends on all of us acting responsibly."
The Labour leader said Mr Johnson was "kidding no-one" after the PM claimed the system was "as good as - or better - than any other system" across the world.
The Labour leader also urged Mr Johnson to "be honest" about mistakes made during the pandemic.
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