Boris Johnson is facing demands to explain whether he delayed the addition of India to the government’s travel “red list” in April because he was hoping his high-profile visit to the country could go ahead.

Travellers from India have been blamed for bringing the highly infectious B1.617.2 strain of Covid-19 to the UK, sparking the current surge in cases in towns such as Bolton, Blackburn and Bedford.

India was placed on the red list – requiring arrivals to quarantine in airport hotels – on 19 April. But the control did not come into effect until 23 April, triggering a rush by thousands of passengers to get in before the deadline. And questions were asked today about why the country did not join the list announced on 2 April along with neighbours Pakistan and Bangladesh, where infection rates were lower.

On 2 April, recorded infections in India were running at about 90,000 a day – or one in every 15,000 of the population – compared to 4,700 in Pakistan (about one in 45,000) and 6,800 in Bangladesh (around one in 24,000).

Health minister Edward Argar said it was impossible to know how many travellers from India in April were infected with the mutant strain, and admitted it was possible that some of them had used public transport to travel from airports in order to undertake the 10-day home quarantine required when the country was on the amber list.

By the time India was placed on the red list, the country was approaching 300,000 cases a day as the world’s worst coronavirus hotspot.

But it was not until 19 April that Mr Johnson cancelled a trip, scheduled for the following weekend, to meet PM Narendra Modi in New Delhi, one of the cities worst hit by the virus.

The visit, already postponed once earlier in the year, was to be Mr Johnson’s first major bilateral overseas visit since becoming prime minister, and a crucial part of his post-Brexit “global Britain” drive to establish new trade links with other parts of the world.

Labour’s shadow home secretary Nick Thomas-Symonds demanded to know whether Mr Johnson put off imposing tighter controls on travel from India because he was hoping, even as the situation in the country deteriorated, that his own trip could go ahead.

“Time and again the UK government have put the safety of the British people at risk by doing too little too late during the Covid pandemic,” Mr Thomas-Symonds told The Independent.

“The PM has serious questions to answer about suggestions that he delayed adding India to the red list until he decided to cancel a scheduled trade visit to India, and that he did not put the safety of the British people first.

“Moving so slowly to add India to the red list has meant that dangerous variants have reached us as a result. The Conservative government must accept the responsibility for this.

“Other countries have acted swiftly to protect their borders against Covid and emerging strains. That’s why Labour has long called for the introduction of a comprehensive hotel quarantine system.”

When health secretary Matt Hancock first announced India was going on the red list, he was challenged over the delay in the House of Commons by home affairs committee chair Yvette Cooper, who told him: “This week, Hong Kong identified 47 covid cases on a single Delhi flight. Before Friday, we still had 16 direct flights from India and many more indirect ones.”

Evidence from Public Health England shows that between 2 and 23 April, cases of the Indian variant were detected in people who had travelled to the UK from variant hotspots Delhi and Mumbai, with the sharpest rises after 19 April, as demand for India-UK flights went through the roof. The first cases of domestic transmission of the strain were also detected during this time.

The chair of the all-party parliamentary group on coronavirus, Layla Moran, 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.”

The Liberal Democrat MP told The Independent: “If they’re going to announce that an area with variants of concern is going on the red list, it needs to be immediate. And they shouldn’t be waiting a number of weeks after a variant of concern is found to put that country on the red list at all.

“The blame for the surge in the Indian variant lies at the door of Boris Johnson.”

And Ms Cooper said: “The Government was warned about the India variant – cases had been rapidly rising there since February – but inexplicably delayed putting India on the red list until 23 April, after many thousands of people had returned from India bringing in many hundreds of new-variant cases.

“When they did finally add India to the red list – two weeks after they’d added Pakistan on 9 April – they gave travellers four days’ notice to rush back. Why didn’t they introduce additional testing for those travellers before they were able to get on public transport home?”

Mr Argar said the decision not to place India on the red list at the start of April was made “on the basis of the evidence”.

Factors taken into consideration included not only infection rates and the emergence of new variants, but also the country’s capacity to carry out genomic sequencing, which is one of India’s strengths.

“On the basis of the advice at the time, the decision was taken to place Pakistan and Bangladesh on the red list at a particular point, and India on that list subsequently,” he told BBC Radio 4’s Today programme.

Mr Argar insisted the UK’s border restrictions are “among the strictest and the toughest in the world”.

Evidence suggests that the “overwhelming majority” of arrivals from amber list countries comply with home quarantine requirements, he said.

There was no “hard and fast answer” on how many people arriving from India during April were infected with the variant, said Mr Argar.

“One person could bring in a variant, and that could transmit quickly,” he said. “It is impossible to completely hermetically seal the borders of a country.

“I do think we’ve got the right border controls in place to do everything we can to minimise – you can never totally eliminate it – but to minimise the risks.”

A government spokesperson said: “We have some of the toughest border measures in the world.

“We took precautionary action to ban travel from India on 23 April, six days before this variant was put under investigation and two weeks before it was labelled as of concern. We have since sped up our vaccination programme and put in enhanced local support to curb transmission.

“Prior to India being placed on the red list in April, anyone coming to the UK had to test negative and quarantine for 10 days.”