The Indian variant could pose “serious disruption” to lockdown easing in England on 21 June, Boris Johnson says.
The PM said if it was found to be “significantly” more transmissible “we are likely to face some hard choices”.
The wait between jabs will be cut from 12 weeks to eight for the over-50s and clinically vulnerable because of concern over the variant, he said.
Cases of the Indian Covid variant have more than doubled in the past week, Public Health England figures show.
Surge testing is already taking place in 15 areas across England, including Bolton, Blackburn, London, Sefton and Nottingham.
According to Friday’s government figures, a further 17 people have died in the UK within 28 days of a positive test and another 2,193 coronavirus cases have been recorded.
Speaking at a Downing Street briefing alongside the UK’s chief medical adviser, Prof Chris Whitty, Mr Johnson said first Covid vaccine doses will also be prioritised “for anyone eligible who has not yet come forward”. Currently, England’s vaccination rollout is open to people over the age of 38.
The prime minister said he did not believe that the “present evidence” showed a need to delay the next stage in the easing of lockdown in England on Monday – which is when people can meet in groups of up to 30 outdoors and six people or two households can meet indoors.
But Mr Johnson warned: “I have to level with you that this new variant could pose a serious disruption to our progress and could make it more difficult to move to step four in June.”
He said he did not believe it was “impossible” to go ahead with step four of England’s roadmap to ease coronavirus restrictions, but “we have to be utterly realistic” and there was “the risk of disruption and delay”.
The government’s aim with the fourth and final step of the roadmap is to remove all legal limits on social contact on 21 June – meaning there will be no limits on gatherings indoors or outdoors – nightclubs will be allowed to reopen and there will be no limits on weddings.
The government has said easing restrictions on 21 June depended on its four tests being met – including that vaccines continue to be effective and the risks are not fundamentally changed by new variants.
Mr Johnson added that there was “no evidence to suggest that our vaccines will be less effective in protecting people against severe illness and hospitalisation”.
The Army would be deployed on the streets of Blackburn and Bolton to give out tests to help the surge testing efforts, he said.
There will be an acceleration of the vaccine rollout there, Mr Johnson said, including longer opening hours at vaccination centres.
Prof Whitty said delays to the vaccination programme for younger people were not expected with the prioritisation of second doses for older age groups.
He warned that if the Indian variant proved to be more transmissible than other variants, the UK could see “a really significant surge” in Covid-19 cases, adding: “That’s a really critical question to which we do not yet have the answer.”
“We expect over time this variant will overtake and come to dominate in the UK in the way that the Kent variant did,” he added.
The Scientific Advisory Group for emergencies has said there is a “realistic possibility” that the variant could be as much as “50% more transmissible” than the Kent strain.
Public Health England figures released on Thursday have risen from 520 cases up to 5 May to 1,313 cases.
Most of the cases – 1,255 – are in England, while there are 35 in Scotland, 12 in Northern Ireland and 11 in Wales, according to PHE.
It said four people have now died with the Indian variant of concern as of 12 May.
Mobile testing units have been set up in Bolton, PHE said, and door-to-door PCR testing had been offered to 22,000 residents. A PCR test is the most accurate way to check for Covid.
More doses have been delivered to the town, which has also set up a vaccine bus to increase uptake among those who are eligible.
Extra clinics will open in Blackburn and Darwen in Lancashire from next week to offer the vaccine to those who are eligible under national guidelines.
But the area’s public health director, Prof Dominic Harrison, said he was “furious” the government had refused a request to extend vaccinations in Blackburn.
On Thursday, the council initially said all over-18s would be offered a jab, before later stating only those with underlying conditions could book an appointment.
Meanwhile, the R number – or reproduction number – in England is estimated to have risen slightly from between 0.8 and 1.0 to between 0.8 and 1.1, according to the government’s Scientific Advisory Group for Emergencies.
If the R number is above one then the number of cases keeps increasing.