People should not get “too carried away” with the easing of lockdown restrictions in England on Monday as it could derail plans to fully reopen the country on June 21, Kwasi Kwarteng has said.

The business secretary warned the Indian coronavirus variant, which is as much as 50% more transmissible than the Kent strain, could delay stage four of the government’s roadmap.

“We’ve got to behave sensibly, we’ve got to exercise some caution because if people get too carried away, we could jeopardise the ability to reopen on June 21,” he told LBC.

“It is fairly clear to me in terms of common sense that what you can do is socialise in a normal way but obviously we advise ordinarily against excessive drinking, endangering people, getting too many large groups together if that can be avoided.

“If we get too carried away and the mutant variant spreads too quickly, that could endanger our ability to open up on June 21.”

Boris Johnson has called for a “heavy dose of caution” as indoor socialising and physical contact are permitted again from today.

Pubs and restaurants will be able to welcome customers back indoors, visits to the homes of friends and family can resume and the foreign holiday ban has ended.

Hugs and other physical contact between households are also permitted for the first time since restrictions began more than a year ago.

Ministers are hoping surge testing and vaccines will allow a safe opening up of the nation, with jabs due to be extended to the over-35s this week.

But health secretary Matt Hancock did not rule out the possibility of imposing local lockdowns in areas such as Bolton to tackle the Indian variant, which he warned could “spread like wildfire”.

He said there are more than 1,300 cases of the Indian variant of concern, which is “relatively widespread in small numbers” and is becoming “the dominant strain” in Bolton and Blackburn.

Offering good news over plans to ease restrictions without unleashing a fresh wave of infections and deaths, Hancock said there is “new very early data” from Oxford University giving confidence that existing vaccines work against the variant.

“That means that we can stay on course with our strategy of using the vaccine to deal with the pandemic and opening up carefully and cautiously, but we do need to be really very vigilant to the spread of the disease,” he told Sky’s Sophy Ridge On Sunday.

“We have a high degree of confidence that the vaccine will overcome.”

Cinemas, hotels and B&Bs can also reopen, with the “rule of six” applying indoors and the order to physically distance having ended between friends and family.

The easing came after official figures showed more than 20 million people have received both vaccine doses, covering more than 38% of UK adults, while more than 69% had received at least one.

Sir John Bell, Oxford’s regius professor of medicine, said the result of lab experiments investigating whether the vaccine neutralises the Indian variant “looks OK”.

“It’s not perfect, but it’s not catastrophically bad,” he told Times Radio, adding there is only “a slight reduction in the ability to neutralise the virus”.

With surge testing also under way in areas of Blackburn, Sefton and London, Government scientific adviser Professor Sir Mark Walport warned it will be “extremely important” to keep an eye on the numbers over the next few weeks.

He told Ridge that “it’s fair to say it is a perilous moment” and said “my personal judgment is that I will do things outside as far as possible”.