The Covid variant first detected in India is set to be the dominant strain in the UK within days, experts have said, with the government and health teams struggling to contain cases, which have risen by more than 75% since Thursday.

With the rapid spread of the more transmissible B.1.617.2 variant threatening to reverse moves to ease lockdown, the government faced intense pressure to more fully explain the delay in adding India to the so-called red list of countries.

Dominic Cummings, Boris Johnson’s former chief adviser, joined the criticism on Monday, calling the UK’s border policy a “joke”.

Johnson is now set to delay plans to announce an end to social-distancing rules, postponing the conclusion of a review expected by the end of the month, casting significant doubt over the wider plan to relax most lockdown rules on 21 June.

Speaking on the day indoor hospitality and other venues were allowed to reopen, Matt Hancock told MPs that 2,323 cases of the variant known as B.1.617.2 had been confirmed, up from 1,313 on Thursday, with 483 of those in the outbreaks in Bolton and Blackburn. There are now 86 local authorities with five or more confirmed cases, he said.

Describing a “race between the virus and the vaccine”, the health secretary rejected calls from Labour to consider a push to vaccinate all adults in the most affected areas, saying that surge testing was the best remedy.

Hancock said 35,000 more tests had been distributed or collected in Bolton and Blackburn, along with a push to target those eligible for vaccinations, with 6,200 jabs carried out in Bolton alone over the weekend.

But new data from the the Wellcome Sanger Institute’s Covid-19 genomic surveillance, which excludes samples from recent travellers and surge testing,has shown how rapidly and widely the variant appears to be embedding.

According to an analysis of the data by Prof Christina Pagel, the director of the Clinical Operational Research Unit at University College London and a member of the Independent Sage group of experts, the variant was detected in almost 30% of Covid samples collected in England in the week ending 8 May.

Paul Hunter, a professor in medicine at the University of East Anglia, said the India variant seemed set to supplant that first detected in Kent, which was in turn notably more transmissible than earlier forms of coronavirus.

“There is no evidence that the recent rapid rise in cases of the B.1.617.2 variant shows any signs in slowing,” he said. “This variant will overtake [the Kent variant] and become the dominant variant in the UK in the next few days, if it hasn’t already done so.”

This has prompted renewed questions about why India was not added sooner to the red list of countries, where all arrivals apart from UK nationals are banned, and those who do come must quarantine in a hotel for 10 days.

Responding in the Commons, Labour’s shadow health secretary, Jonathan Ashworth, said UK borders “have been as secure as a sieve”.

Yvette Cooper, the Labour chair of the Commons home affairs committee, said people would “understandably feel angry” if reopening from lockdown was put at risk due to border decisions, citing statistics showing that in early April, arrivals from India tested positive for Covid at 50 times the then UK rate.

Separately, in a Twitter thread prior to his appearance before the Commons health and science committees on 26 May, Cummings lambasted the UK’s response to Covid, citing as an example “our joke borders policy”.

In a sentiment likely to raise some eyebrows given his own long-distance drives last spring, Cummings also argued that lockdowns only worked with “serious enforcement”.

Hancock defended the government’s approach in the Commons, saying it had added India to the red list on 23 April, six days before the B.1.617.2 variant was put under investigation and two weeks before it was labelled as being of concern.

However, another variant first discovered in India and closely related to the variant of concern, called B.1.617.1, was was designated under investigation on 1 April, weeks before travel from India was restricted.

Ministers could face a significant backlash if the spread of the B.1.617.2 variant derails the planned June reopening, or even forces the reversal of some of Monday’s changes.

A government source said more time was needed to gather data about the effect of the variant, but stressed it did not necessarily mean the 21 June date would slip. “We thought we would be in a position to give some notice well in advance, now we will need a bit more time,” the source said.

The review into social distancing was expected to announce an end to the 1-metre rule for hospitality venues, which has seen some needing to significantly reduce capacity, as well as an end to fines for not wearing masks.

In a sign that the UK government could be preparing ground for its roadmap to slip, a Downing Street spokesman said: “The variant first identified in India could pose serious disruption to this progress, and could make it more difficult to move to step 4.

“Our decision will be based on the very latest data, and we want to allow as much time as possible to assess this so we will set out plans as soon as the data allows.”

Some Conservative MPs have expressed grave worries at the idea of a reversal to reopening.

“When will this government actually take a little bit of risk and allow people to get on with their lives again?” Huw Merriman, the Tory MP who chairs the transport committee, told Hancock in the Commons.

Giles Watling, who represents the seaside resort of Clacton in Essex, said that local businesses had reported a boom in bookings as people planned UK holidays this summer, and there was huge worry about the idea of new restrictions.

“I understand that ministers have to act, but we didn’t really get a summer season last year, and it would be a real kick in the teeth if we couldn’t open up in the way we have been planning.”

Peter Walker, Nicola Davis and Jessica Elgot