"I think it is sensible to wait just a little longer," Mr Johnson told a news conference on Monday evening. "As things stand, and on the evidence that I can see right now, I'm confident that we will not need more than four weeks."
Announcing the delay, he said government was “so concerned” by the Delta variant that is “now spreading faster than the third wave predicted in the February roadmap”.
“We’re seeing cases growing by about 64 per cent per week, and in the worst affected areas, it’s doubling every week,” he added.
“And the average number of people being admitted to hospital in England has increased by 50 per cent week on week, and by 61 per cent in the North West, which may be the shape of things to come.
“Because we know the remorseless logic of exponential growth and even if the link between infection and hospitalisation has been weakened it has not been severed.”
He hopes deaths will be significantly reduced by mid-July because two-thirds of adults will have then been offered both vaccine doses due to the delay being coupled with a reduction in the time between jabs for the over-40s.
Limits on numbers for sports events, pubs and cinemas will therefore remain in place, nightclubs will stay shuttered and people will be asked to continue working from home where possible.
"It's unmistakably clear the vaccines are working and the sheer scale of the vaccine roll-out has made our position incomparably better than in previous waves," he said.
"But now is the time to ease off the accelerator, because by being cautious now we have the chance in the next four weeks to save many thousands of lives by vaccinating millions more people.
“And once the adults of this country have been overwhelmingly vaccinated, which is what we can achieve in a short space of time, we will be in a far stronger position to keep hospitalisations down, to live with this disease, and to complete our cautious but irreversible roadmap to freedom.”
However the Prime Minister did announce a lifting of the 30-person cap on wedding guests and wakes.
And he said pilot events such as Euro 2020 and “some theatrical performances" would continue, indicating they will be allowed larger crowds than under the restrictions currently in place elsewhere as part of the research programme.
There will be a review of the measures in two weeks’ time on June 28, Mr Johnson said. If the data is much better than expected, the Government could ease restrictions earlier, although a Downing Street spokesperson said that was unlikely.
When the PM initially outlined his roadmap out of lockdown in February this year, it was hoped that all limits could be removed by June 21.
The aim was to reopen almost all of the economy, including sectors that remained shut down.
However, the variant that emerged in India has thrown the final step out lockdown off course.
The move is set to spark fury from backbench Conservative MPs who warn about the impact on the economy and peoples’ mental health.
On weddings, there will be no limit on the amount of guests allowed to attend a ceremony.
However, couples will only be able to invite guests based on their venue’s ability to socially distance and the provision of table service.
Guests will also have to sit on tables of no more than six people and singing and dancing indoors will still be banned.
Although, officials conceded that outdoors on private land the no-dancing message will be guidance rather than an outright ban.