Professor Peter Openshaw said his fellow scientists were “very concerned” after a cluster of cases of the South African Covid-19 variant were found in London.
Some 44 confirmed cases of the variant have been found in Lambeth and Wandsworth, with a further 30 probable cases identified, the Department of Health and Social Care (DHSC) said.
Surge testing for those who live, work or travel through those areas is being rolled out, while NHS Test and Trace is providing additional testing in an area of Southwark where a case linked to the other cluster has been identified.
Watch: 'Biggest surge testing operation to date' under way as dozens of South African variant cases found in south London
Prof Openshaw, a member of the Covid-19 clinical information network, told BBC2's Newsnight: "A lot of we scientists are very concerned about what's happening at the moment.
"I think we're all just hoping that the staged reduction in lockdown is going to be OK. It is being done reasonably cautiously but I think this is not good news.
"If we get rapid spread of the South African or other more resistant variants, it may well be that we are going to have to put the reductions of lockdown into reverse."
According to government figures, there have been 533 genomically confirmed cases of the South African variant in the UK and another 11 probable cases.
Lockdown was further eased in England on Monday, with non-essential retail and pub beer gardens allowed to reopen – and prime minister Boris Johnson said there were no plans at present to change the road map out of lockdown.
The next "waymarks" on England's plan to ease restrictions are due on 17 May and 21 June.
Meanwhile, researchers are aiming to recruit more than 1,000 people who have already had one Covid vaccine to test the efficacy of mixing different types of jab.
The Com-Cov2 trial, which is being led by the Oxford Vaccine Group at the University of Oxford, will look at how well people's immune systems respond when the booster dose is a different type to their first vaccination.
Additional reporting by PA
Watch: How England will leave lockdown