Plans to further lift coronavirus in restrictions in June could be delayed by the spread of a new more virulent variant from India, the government has indicated.
It comes as a leading viral disease expert raised doubts over whether restrictions could be eased in line with the government's roadmap.
Professor Paul Hunter from the University of East Anglia's Norwich Medical School said step four was "in doubt" and there could be a "huge number of cases" by June at the current rate of increase.
"I think the big question is how many of people who are getting the Indian variant will end up requiring hospitalisation," he told BBC Radio 4.
"And at the moment the hospitalisation rate doesn't seem to be increasing yet, although if this becomes much more common we'll almost certainly see some increase, so I think it's certainly a concern.
"I think the step four is in doubt in June now, but we really need to see what impact it has on severe disease before we can really be certain."
Under step four all legal restrictions on social contact would be removed - reopening nightclubs for the first time since the start of the pandemic, and lifting restrictions on live events and performances.
Public Health England has recorded 1,313 UK cases of the India variant, with the Thursday figures more than double the 520 cases recorded up to 5 May.
The Department of Health and Social Care (DHSC) has said there is "no firm evidence yet to show this variant has any greater impact on severity of disease or evades the vaccine".
But ministers have suggested vaccinations could be brought forward and reallocated to the worst-hit areas to try to minimise the spread and impact of the variant.
I think the step four is in doubt in June now, but we really need to see what impact it has on severe disease before we can really be certain
Professor Paul Hunter, UEA
And a minister on Friday morning said new local lockdowns or further national restrictions were on the table to control the situation.
Vaccines minister Nadhim Zahawi on Friday morning told LBC radio: “We have got to break the cycle of infection, because one of those big tests was infection rates have to be suppressed, and the other big test is variants.
“If those cause a problem, then the tests will fail. The four tests have to be met for June 21.”
Asked why a delay in lifting restrictions could be necessary, Professor Hunter added: "Well, because if the epidemic continues to increase, if the Indian variant of the epidemic continues to increase at the same rate as it has over recent weeks, we're going to have a huge number of cases by June.
"The issue though is that because it seems to be spreading in unvaccinated younger people at the moment and not yet that much more active in older people maybe we'll be able to weather it and we'll still be able to have the step four in June.
"But if that increases cases in elderly and starts to increase hospitalisations, and puts pressure on the NHS again then I think step four would be in doubt."
The disease expert cast doubt on whether reallocating vaccines would be effective, stating that the "downside" was "who do you take the vaccines from".
"It does take a couple of weeks to work, so if you're moving vaccines away from areas where they currently don't have much Indian variant and that is increasing, by the time you start getting round to vaccinating that group again when maybe the epidemic, the Indian variant, is increasing rapidly again, probably you might well have been able to stop that if you hadn't diverted vaccine to surge areas," he explained.
Asked whether local lockdowns as seen last year were being considered, Mr Zahawi said "we will take nothing off the table".
He told BBC Breakfast: "Over a year of dealing with this pandemic suggests that the most effective way of dealing with this, because we have had such a successful vaccination programme, is the surge testing by postcode, the genome sequencing and isolation, so that is our focus, that is our priority.
"But we will take nothing off the table, whether it is regional or national further measures that we would need to take, we will deal with this."