Contingency plans are being drawn up to delay the easing of restrictions on June 21 by two weeks if hospitalisations rise.
Officials are said to be working on back-up options if Boris Johnson decides to postpone the final stage of the lockdown roadmap.
Plans to scrap remaining lockdown measures are on a knife-edge amid mounting concern from scientists over the spread of the Delta variant – formerly known as the Indian strain.
Whitehall sources told the Daily Mail that fall-back proposals were being drawn up if the Prime Minister decides to slam on the brakes when he updates the public on June 14.
Mr Johnson has insisted that there is “nothing in the data at the moment” to delay Step 4 of the roadmap – but his tone has been increasingly cautious in recent days.
And Health Secretary Matt Hancock warned on Thursday it remains “too early” to say whether all restrictions can end on June 21 as planned.
Speaking ahead of a G7 health ministers’ meeting, he said: “It’s too early to say what the decision will be about step four of the road map, which is scheduled to be no earlier than June 21.
“Of course I look at those data every day, we publish them every day, the case numbers matter but what really matters is how that translates into the number of people going to hospital, the number of people sadly dying.
“The vaccine breaks that link – the question is how much the link has yet been broken because the majority of people who ended up in hospital are not fully vaccinated.
“That’s a good sign if you like because it means that the vaccine is clearly protecting people from ending up in hospital but it also demonstrates that we need to keep going with this vaccine programme.”
Remaining restrictions on social contact would be scrapped and shuttered businesses such as nightclubs would reopen on June 21 under the PM’s lockdown roadmap.
Officials are also reviewing whether social distancing measures such as masks and the ‘one-metre plus’ rule can be axed.
Ministers are pinning their hopes on the vaccine being able to see off the threat of the new variant.
Three quarters of British adults have had their first dose of the jab – but a recent study by Public Health England found the vaccine was only 33% effective against the Delta strain at first dose.
The Pfizer jab was 88% effective after two doses, while the AstraZeneca vaccine was 60% effective.
Half of Brits have had both doses of the jab.
New Test and Trace figures show the number of people testing positive for Covid-19 in England has risen by 22%, reaching the highest level in six weeks.
Some 17,162 people had a positive test at least once in the week to May 26, a rise of more than a fifth on the previous week.
It marks the highest number since the week to April 14.