Most current coronavirus rules will remain in place for another four weeks after the planned 21 June unlocking, government sources have told the BBC.
Senior ministers have signed off on the decision to delay the lifting of all legal restrictions on social contact.
That could mean capacity limits for sports, pubs and cinemas will remain, and nightclubs would stay closed.
Prime Minister Boris Johnson is due to confirm the delay later at a news conference.
The extension will be put to a Commons vote this month and could trigger a sizeable Conservative backbench rebellion.
Stage four of the government's roadmap out of lockdown would see all legal limits on social contact removed.
But many scientists have called for the reopening to be delayed to enable more people to be vaccinated and receive second doses amid rising cases of the Delta variant.
A delay would also allow for more work to be done on whether vaccines are breaking, or simply weakening, the link between infections and hospitalisations.
Health Minister Edward Argar told BBC Breakfast that he could not confirm the delay ahead of the prime minister's announcement, but that there was a "concerning increase" in cases of the Delta variant and numbers in hospital were "beginning to creep up".
Most severe cases were among unvaccinated people or those who had only one dose, he said. He said at the current rates of vaccination, over four weeks they would be able to administer nearly 10 million second doses to increase protection.
Mr Argar said the prime minister would address issues of economic support in any announcement and said he was "very sensitive" to the situation of couples who have already had to postpone their weddings, in some cases multiple times.
All areas of Scotland are due to move to Level Zero Covid restrictions on 28 June - meaning bigger groups can gather in cafés, pubs and restaurants, although they will still have to observe social distancing.
Limits on indoor gatherings in Northern Ireland are scheduled to be relaxed on 21 June - when the current rules in Wales will also be reviewed.