British Prime Minister Boris Johnson said the government would accelerate its COVID-19 vaccination programme, to try to contain a fast-spreading variant first identified in India that could knock a reopening of the economy off track.
The UK has delivered one of the world's fastest inoculation campaigns, giving a first shot to almost 70% of the adult population and a second to 36%, helping to reduce infection rates and deaths.
But the emergence of the B.1.617.2 variant in parts of northern England and London has prompted some scientists to call for the reopening to be delayed, and a rethink on the speed of the vaccine rollout.
"I believe we should trust in our vaccines to protect the public whilst monitoring the situation very closely because the race between our vaccination programme and the virus may be about to become a great deal tighter," Mr Johnson told a news conference.
He said the government would accelerate remaining second doses to the over 50s and those clinically vulnerable to just eight weeks after the first dose, and would prioritise first doses for those eligible who had not yet come forward.
Even so, the spread of the variant could disrupt Britain's progress out of lockdown, making it more difficult to move to the final stage of a staggered reopening of the economy in June, he said.
He said the variant could "pose a serious disruption" to plans to ease restrictions and "could make it more difficult" to end them as hoped in June.
Mr Johnson had aimed to lift all restrictions on 21 June, after allowing people in England from Monday to hug again, meet in small groups indoors and travel abroad.
Chris Whitty, England's chief medical officer, said there was now confidence that B.1.617.2 was more transmissible than the "Kent" variant that fuelled England's second wave of infections. He said B.1.617.2 could come to dominate in Britain.
There is a "realistic possibility" that the Indian coronavirus variant could be as much as "50% more transmissible" than the Kent strain, the British Scientific Advisory Group for emergencies (Sage) has said.
The minutes of yesterday's meeting between the UK government's scientific advisers said that it is "highly likely that this variant is more transmissible than B.1.1.7 (high confidence), and it is a realistic possibility that it is as much as 50% more transmissible".
Public Health England said yesterday there had been 1,313 cases in England of B.1.617.2 in a week, more than double the previous week's figure, with four confirmed deaths.
Dr Whitty said so far there had not been a significant increase in hospitalisations from the variant, which may be because more people had been vaccinated.
But both Mr Johnson and Mr Whitty said it was still early days, and scientists would need to scrutinise data over the next two or three weeks to truly see the impact of the variant.
Britain put India on a travel "red list" in April, meaning all arrivals from India - now suffering the world's worst wave of COVID-19 - would have to pay to quarantine in a government-approved hotel for 10 days.
Media reports at the time suggested that, because the quarantine requirement was announced four days in advance, many people had sought to fly beforehand. Britain has a large South Asian community.
Even with new variants, the government is likely to want to avoid repeating the regional curbs used last year, which ultimately failed to prevent two further national lockdowns.
Additional reporting PA