Boris Johnson has said the government must “be cautious” as it decides whether to end all Covid restrictions in England on 21 June.
Cases are rising as the Delta variant spreads, with details of the next step on the “roadmap” due on Monday.
The prime minister told the BBC it was important any easing of restrictions was “irreversible” and warned that hospitalisations were increasing.
The BBC has been told a delay of up to four weeks is being considered.
Senior medical advisers have urged a brake to the final easing of lockdown restrictions, which, if it went ahead in full, would mean removing limits on social contact and the number of people allowed into homes.
Pubs, clubs, theatres and cinemas would also be allowed to operate without capacity limits, with sports events taking place in full stadiums, nightclubs reopening and limits of the number of wedding guests going.
Asked by BBC political editor Laura Kuenssberg whether all restrictions would now be lifted, Mr Johnson said people would have to wait until Monday to find out
What I can certainly say is we are looking at the data, continuing to do that, but what you can certainly take is… the roadmap was always cautious but irreversible, and in order to have an irreversible roadmap, we’ve got to be cautious.”
It is often said the vaccination programme has broken the link between infections and hospitalisation.
But in truth it has just weakened it. During winter about one in 10 Covid infections went on to become a hospital admission. Now with admissions rising early data suggests it may be close to one in 20.
If that’s right and the trajectory in terms of increasing infections continues we could see 2,000 admissions a day within two months. That’s twice what the NHS would normally see for all types of respiratory infections in a bad winter.
But that’s a lot of ifs – and the data on its own doesn’t tell the full story. More admissions seem to be among younger people this time who are less sick.
That makes sense as very low numbers of the double-jabbed are ending up in hospital.
There’s a lot of uncertainty. In a few weeks scientists will be able to give ministers a clearer idea of the cost of a full unlock so it can be balanced against the benefits. What’s more, the number of vaccinations done would be higher, providing even more protection.
The Delta variant, first identified in India, now accounts for nine in 10 UK coronavirus cases.
It is roughly 60% more transmissible than the Alpha – or Kent – variant, Public Health England data suggests.
The government put India on the “red list” of places to which travel is banned on 23 April, later than countries such as Pakistan and Bangladesh.
Asked if he regretted this, the prime minister said that, at the time, it had not “even been identified as a variant under investigation, let alone identified as a very different concern”.
“Pakistan had about 13 times as many variants of concern or variants of interest as as India at that stage,” Mr Johnson said.
“Yet we put India on the red list on 23 April, which was days before it was even identified as a variant under investigation.”
Mr Johnson, who is currently hosting the G7 summit in Carbis Bay, Cornwall, said the government had made “a massive progress with the vaccination programme”.
“It’s done a huge amount of good – but you have got to be cautious to be irreversible,” he added.
“Again you have hospitalisations up, you’ve got cases up. There are grounds for caution. That doesn’t mean that this country hasn’t made enormous progress with vaccination.
“Clearly what we have is a race between the vaccines and the virus – and the vaccines are going to win. So it’s just a question of the pace.”
A further 8,125 confirmed coronavirus cases in the UK were announced by the government on Friday.
With this, and the spread of the Delta variant in particular, in mind, the British Medical Association has joined public health officials in calling for a delay to ending restrictions.
BMA council chairman Dr Chaand Nagpaul said: “It’s not just about the number of hospitalisations, but also the risk to the health of large numbers of younger people, who can suffer long-term symptoms.”
The government said in February that England’s restrictions would only be lifted if:
the vaccine rollout continues successfully
evidence shows vaccines effectively reduce 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 29 million people in the UK have had both doses of a vaccine – 55.4% of the adult population.
The R number in England is estimated to have risen to between 1.2 and 1.4, meaning that, on average, every 10 people with Covid will infect between 12 and 14 others.
The easing of restrictions is happening at different speeds across the other nations of the UK with 21 June being given as an “indicative date” for a number of changes in Northern Ireland, including 10 people from two households being allowed to meet in private homes, up to 15 people being able to meet outdoors in a garden and theatres and concerts able to restart.
Lockdown measures in Wales will be reviewed on 21 June, while the whole of Scotland is due to move to level zero on 28 June – but the prevalence of the Delta variant may delay that.