The Indian virus variant could ทางเข้า slotxo pose "serious disruption" to lockdown easing in England on 21 June, the PM says.
Boris Johnson said if it was found to be "significantly" more transmissible there could be "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 added.
Cases of the Indian coronavirus variant have nearly tripled 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 - when pubs and restaurants will be allowed to serve customers indoors and six people or two households can meet in a private home.
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."