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.