A rise in cases of the coronavirus variant dominant in India could "pose serious disruption" to Britain's reopening plans, Prime Minister Boris Johnson warned Friday.
England will take the next step of reopening on Monday as planned, but the final stage, currently scheduled for June 21, could be in doubt.
"I do not believe we need to delay our roadmap," said Johnson.
But, "this new variant could pose a serious disruption to our progress," he said, adding "We will do whatever it takes to keep the public safe."
The health ministry said the B1.617.2 variant is "beginning to spread increasingly rapidly" in northwest England and to a lesser extent in London, "and decisive action is being taken to further control its spread".
Second doses of vaccines will be accelerated for the over-50s and the clinically vulnerable in a bid to keep the strain at bay, Johnson told the press briefing.
Surge testing and possible local restrictions are also in the mix after infections of the variant rose from 520 last week to 1,313 this week, officials said.
The government is waiting on data that will indicate if the new variant is more transmissible than other strains currently circulating before deciding on its next step, said Johnson.
Scientists believe it is more transmissible, but are unsure of by how much, revealed Chief Medical Officer Chris Whitty.
If only slightly, the country will reopen as planned, said Johnson.
But if there is evidence of uncontrollable spread and the threat of increased pressure on NHS then more restrictions are likely, he added.
Trust our vaccines
Britain has driven down cases over the last few months thanks to a successful vaccine campaign, and officials are optimistic they will be effective against the variant.
"So far, there is no evidence that our vaccine will be less effective against serious illness and hospitalisations," said the prime minister, adding the country was "in a different position from the last time we faced a new variant."
"We should trust in our vaccines... while monitoring the situation very closely," he said.
The government last month added India, which is experiencing a devastating wave of deaths from Covid-19, to a travel red list, meaning travellers from the country have to quarantine in hotels on arrival in Britain.
In the northwest English city of Bolton, which has a sizeable population of South Asian origin, mobile testing units have been deployed and door-to-door testing is also on offer.
Britain has been gradually reopening its economy, after months of coronavirus restrictions.
On Monday, indoor mixing will resume in England, including in pubs and restaurants.
Deepti Gurdasani, senior lecturer in epidemiology at Queen Mary's University London, said that must now be postponed.
"Further, we may have to actually strengthen restrictions a bit more to get on top of this, because we need to remember this is actually growing with current restrictions in place and growing rapidly," she told Times Radio.
The variant dominant in India is also triggering concern in Scotland and Wales, which administer their own health policy.
Scotland announced on Friday it was going ahead with its next stage of reopening on Monday, except in Glasgow, where restrictions will remain unchanged due to a rise in cases of the variant.