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.
'Break clause'
Former Conservative cabinet minister Damian Green told the BBC's Westminster Hour that he wanted to see the decision to delay reviewed as more data becomes available.
"I think if it is as long as a month then there should be a break clause after two or maybe three weeks, to say that if we can tell by then that the rise in cases is not leading to a sort of rise in the serious illness that sends people into hospital, then we can unlock earlier," he said.
On Sunday, Foreign Secretary Dominic Raab told the BBC's Andrew Marr Show the government "don't want to yo-yo in and out of measures".
He said the decision on postponing unlocking would depend on whether the link between infections and hospital admissions had been severed.
Under stage four of the roadmap, venues and events would be allowed to operate without capacity limits and the cap on guests at weddings would also be lifted.
Even if these restrictions are removed, rules on face coverings and social distancing could remain in place.
The prime minister has previously said the government must be cautious so any easing of restrictions is irreversible.
Rising infections in the UK are being driven by the Delta variant, first identified in India, which now accounts for 90% of infections.
It is believed to be around 60% more infectious than the Alpha variant - which was first identified in Kent and was previously dominant in the UK - and twice as likely to result in infected people being hospitalised.
Prof Andrew Hayward from University College London, who is a member of the Sage group which advises the government, said easing more restrictions would "fan the flames" of rising infections.
On Sunday, the UK recorded 7,490 new cases of Covid-19 and eight deaths within 28 days of a positive test.
The seven-day average for cases in the UK is up 49% compared with the seven days before.
The government has set out four tests that must be met for the next stage of easing restrictions to go ahead:
• The vaccine deployment programme continues successfully
• Evidence shows vaccines are sufficiently effective in reducing hospitalisations and deaths in those vaccinated
• Infection rates do not risk a surge in hospitalisations which would put unsustainable pressure on the NHS
• Its assessment of the risks is not fundamentally changed by new variants of concern
More than 41 million people in the UK have had a first dose of a coronavirus vaccine, while nearly 30 million have had two doses.
The Night Time Industries Association has warned any delay beyond 21 June would be "catastrophic" for sectors like nightclubs which have been forced to close since March last year.
It said businesses had already spent millions preparing to reopen on that date.
The UK Weddings Taskforce, an industry group, estimates that 50,000 weddings planned in the four weeks from 21 June could be cancelled if the lifting of restrictions is pushed back and that the industry would lose £325m for every week of delay.
Currently the number of guests allowed at weddings and other life events is limited to 30.
Some Conservative MPs also oppose a delay to easing restrictions, including members of the Covid Recovery Group of backbenchers.
The group's chairman, former minister Mark Harper, said any postponement would be a "political choice".
He warned that if the unlocking did not go ahead as planned, restrictions could carry on through the autumn and into the winter as other respiratory infections picked up.
"The effectiveness of our vaccines at preventing hospitalisation means unlocking on 21 June could proceed safely. Any decision to delay will be a political choice," he said.
"Variants and mutations will appear for the rest of time. We have to learn to live with it.
"If our very effective vaccines cannot deliver us freedom from restrictions, then nothing ever will."
Steve Baker, the CRG deputy chairman, questioned how long the country could "fumble along" with restrictions that had such "devastating consequences" for both business and people's mental health.
Labour's Emily Thornberry said the decision over stage four of the roadmap was "the last chance" for the government to follow the science and communicate clearly with the public.
Source: BBC