Boris Johnson has warned that England’s lockdown easing plans may have to be delayed because of the emergence of a new Covid-19 variant that has led to a surge of infections. The UK prime minister said on Friday that infections from the variant known as B. 1.617.2, which was first discovered in India, are spreading rapidly in Bolton, Birmingham, Worcestershire and parts of London.

Infections more than doubled in these areas in the week ending May 13 and four deaths have been linked to the new variant. There is no evidence, however, that it is resistant to vaccines.

Speaking at a Downing Street press conference, Johnson confirmed that a significant easing of England’s lockdown restrictions would go ahead on May 17, with pubs and restaurants reopening for indoor service.

In Scotland, Glasgow will not see restrictions eased because of a rise in cases, while concerns about the new variant also prompted Wales to delay changes to social-distancing rules outside of households.

Johnson warned that the next stage of relaxation in England, due on June 21 — when all existing rules are set to fall away — may now be at risk depending on how infectious the new strain of the virus proves.

“I do not believe on present evidence that we need to delay our road map,” he said. “But I have to level with you that this could be a serious disruption to our progress and could make it more difficult to move to step 4 in June,” he said, adding that the June 21 easing could still happen as planned.

In an effort to combat the rise in infections, the UK will shorten the gap between first and second doses of vaccines from 12 to eight weeks for over-50s and the clinically vulnerable. First doses will also be prioritised for everyone over 40, while surge testing has been introduced to hotspots, as well as new vaccination centres.

Johnson urged caution ahead of Monday’s lockdown easing, asking people to “think twice” about travelling to areas with rising cases. But he also stressed that “infections, deaths and hospitalisations nationally remain at their lowest levels since last summer”.

Prof Chris Whitty, England’s chief medical officer, said there was a “very clear view” that the B. 1.617.2 variant was transmitting more quickly. “We expect over time this variant will overtake and come to dominate in the UK in the way that the Kent variant did.”

Documents released by the UK’s Scientific Pandemic Influenza Group on Modelling (Spi-M) suggested that B.1.617.2 may be “more than 50 per cent” more transmissible than the dominant B.1.1.7 variant, which originated in Kent, but it said more data was required to make a definitive conclusion.

Although the prime minister said the UK “just needs a bit more time to see how that pans out” before deciding on whether the June lockdown easing needed to be delayed, he warned that “some hard choices” would be required if it were to prove significantly more transmissible.

A leading body representing health leaders has called for more clarity from the government and its scientific advisers about the dangers of the variant, warning the new strain could lead to large numbers falling sick, once again putting pressure on the NHS.

Chris Hopson, chief executive of NHS Providers, said: “Given the combination of potentially lower vaccine efficacy, seemingly much faster transmission of the virus and the fact that many people are still unvaccinated, there is a risk that this variant could lead to a large group of people becoming ill with Covid-19 putting significant pressure on the NHS once more.”

Prof Kate Ardern, director of public health for Wigan, said infection levels were far lower in the over-60s — of which large numbers have been immunised — suggesting existing vaccines were effective against the new strain.

She added there was no sign yet that the outbreaks were leading to higher numbers of hospital admissions, although both ambulance call-outs and the number of people seeking help from GPs had risen: “There is demand in the system but it is not yet putting pressure on secondary-care admissions.”

Jonathan Ashworth, Labour’s shadow health secretary, blamed Johnson for failing to close borders quickly enough, and called for vaccinations in the areas of concern to be sped up.

“We now need ‘surge vaccinations’ in areas of prevalence, enhanced contact tracing and, crucially, to fix sick pay and isolation support. Having come so far we don’t want to be set back now,” he said.