The end of the lockdown next month has been plunged into jeopardy after Boris Johnson warned the surge of the Indian Covid-19 variant had created “the risk of disruption and delay”.

In a sharp U-turn – four days after telling the public the end is in sight – the prime minister served notice that he will slam on the brakes if the strain proves to be far more infectious, as feared.

Mr Johnson announced step 3 of his roadmap will go ahead as planned on Monday, allowing pubs and restaurants to serve customers indoors and families to hug.

He also slashed the wait-time for second doses for over-50s from 12 weeks to 8 weeks – defying local leaders in hotspot areas in Lancashire who are demanding the freedom to jab all over-18s.

But, on the final step planned for 21 June, the prime minister warned: “There is now the risk of disruption and delay – delay to that ambition and we have to be utterly realistic about that.”

He also appeared to dash hopes that many more countries will be added to the quarantine-free “green list” for overseas travel in the coming weeks.

“I don’t expect that we will be adding to it very rapidly,” Mr Johnson said. “We will be maintaining a very, very tough border regime for the foreseeable future.”

Minutes after the comments, the Sage advisory group revealed the “realistic possibility” that the Indian variant could be as much as “50 per cent more transmissible” than the Kent strain.

They also sparked fierce criticism of the failure to lockdown the UK’s airports to arrivals from India until many weeks after the first alarms about the new variant.

“Tonight’s news brings into sharp focus Boris Johnson’s reckless failure to protect our borders in this crisis,” said Jonathan Ashworth, Labour’s shadow health secretary.

And Layla Moran, chair of the all-party parliamentary group on coronavirus, said: “Boris Johnson must take responsibility for the failure to prevent the Indian variant taking root in the UK.

“Once again the government acted too late, and the country is sadly paying the price.”

Scientists do not know for certain if the Indian variant has significantly increased transmissibility – or to what degree it eludes the immunity to the virus provided by vaccines.

Mr Johnson said the next few weeks will be vital, as he told the public: “This doesn’t mean that it’s impossible that we’ll be able to go ahead with step 4.”

He denied he had fatally dragged his heels on closing the border to flights from India, insisting: “At that stage, India was not identified as having a variant of concern.”

Mr Johnson stressed there was “no evidence to suggest that our vaccines will be less effective in protecting people against severe illness and hospitalisation.”

Chris Whitty, the chief medical officer, warned the Indian variant was poised to replace the Kent variant – which caused the devastating Covid surge after Christmas – as the most common strain.

“We expect that, over time, this variant will overtake and come to dominate in the UK,” he said.

He defended the decision not to bend the rules to vaccinate younger adults in hotspots – as demanded by officials in Lancashire and by Andy Burnham, the Greater Manchester mayor.

This would lead to “a net disadvantage”, he argued, because people who have just reached adulthood are at far less risk of serious illness or death.

“The sensible thing to do is to prioritise the vaccines to the people most at risk,” Professor Whitty said.

The vaccine rollout will be accelerated in the hotspots of Blackburn and Bolton, including longer opening hours at vaccination centres.

The army will also be deployed on the streets giving out tests to accelerate the surge testing drive.