Lifting the last Covid restrictions in England on 21 June should be delayed to "stop us going backwards", public health officials have said.
Unlocking then would risk an increase in hospital admissions, the Association of Directors of Public Health said.
The government is expected to announce on Monday whether it will remove the last of the restrictions a week later.
Minister Nadhim Zahawi said it had to be careful not to squander the progress made in tackling the virus.
The final stage of lifting restrictions would see all legal limits on social contact removed. Nightclubs would reopen, and restrictions on performances, weddings and other life events would also be lifted.
But concerns about the spread of the Delta variant, first identified in India and now dominant in the UK, have led some scientists to call for a delay to ending lockdown.
Public Health England (PHE) estimates the variant is roughly 60% more spreadable than the previously dominant Alpha variant, first identified in Kent, and could make people more seriously ill.
The latest official data suggests there have been nearly 30,000 new UK cases of the Delta variant in the past week, accounting for 90% of cases.
PHE said nearly two thirds of people infected with the Delta variant have not had a vaccine at all.
Another 8,125 virus cases were recorded in the UK on Friday, according to official figures, and another 17 people died within 28 days of a positive test.
Association of Directors of Public Health vice-president Jim McManus told Radio 4's Today programme the government had a "fiendishly difficult decision" to make on whether to ease restrictions further.
However, he said if we "invest that little bit of time to keep us going forwards, it will stop us going backwards".
"If you get enough people infected, you will get a rise in hospitalisations," he said.
He added that more people getting infected would allow more variants to develop, which could risk a variant developing that evades the vaccine completely.
"So actually, investing a bit of time is really important to enable the vaccine programme to finish and do its job."
Asked about reports in the Times that the government was considering a four-week delay, he said that would be "really welcome".
Latest government figures show that more than 29 million people in the UK have had both doses of a vaccine - 55.4% of the adult population.
Asked about the government decision on 14 June, Mr Zahawi, the vaccines minister, told BBC Breakfast: "We have to be really careful so that those hard won wars against the virus are not squandered."
He reiterated that the government would "share the data on Monday".
"We all know the virus hasn't gone away, it will attempt to mutate," he said. "The Delta variant is more infectious and more severe for those it infects. So, we have to be really careful."
Meanwhile, the latest R number - or reproduction number - in England is estimated to have risen to between 1.2 and 1.4, up from between 1 and 1.2 last week, according to the government's Scientific Advisory Group for Emergencies.
It means that, on average, every 10 people with Covid will infect between 12 and 14 others.
If the R number is above 1.0, the number of cases is increasing.
Mr Zahawi said that the UK was on track to offer all those aged over 50 - who had already had a first jab - a second dose by 21 June.
He also said no-one in the UK would have to wait longer for a vaccine because of the country's contribution to the global vaccination effort.
The UK has pledged to start donating vaccines to poorer countries in the next few weeks, as part of 100 million surplus doses which will be delivered in the next year.
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