No 10 has defended its decision not to ban travel from India sooner, amid concern that the coronavirus variant first discovered there is now spreading quickly in parts of the UK.
India was reporting more than 100,000 cases a day by 5 April, but was not added to the red list until 23 April.
The government said the UK has "some of the toughest border measures".
It comes after the British Medical Association raised concerns about Monday's relaxation of Covid rules.
Prime Minister Boris Johnson said on Friday that the B.1.617.2 Indian variant could pose "serious disruption" to the final stage of lockdown easing in England on 21 June - but insisted Monday's easing would go ahead as planned.
Scientists advising the government are confident the Indian variant spreads more easily, with cases of it nearly tripling to 1,313 in the past week in England.
Asked why the borders were not closed sooner, a government spokesman told the BBC: "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.
"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."
By the time the travel ban came into force, daily Covid cases in India had risen above 330,000.
Surge testing is now taking place in targeted areas across England where virus variants have been found, including postcodes within Bolton, Blackburn with Darwen, several London boroughs, Sefton, Worcestershire, and Nottingham.
Despite concern about the Indian variant, coronavirus restrictions across England, Scotland and Wales are due to be relaxed from Monday.
Curbs lifting across the devolved nations vary, but there will be a greater degree of indoor mixing allowed and more hospitality venues will be able to reopen.
Northern Ireland is due to review its restrictions later this month.
On Saturday, the British Medical Association (BMA) said it had serious concerns about the decision to continue with the easing of lockdown restrictions.
"It is a real worry that when further measures lift on 17 May, the majority of younger people, who are often highly socially mobile and could therefore be most at risk of a more infectious strain, are not yet vaccinated," said the BMA's Dr Richard Jarvis.
Minutes from a meeting of government scientific advisers, held on Thursday, said that "an even faster increase can be expected if measures are relaxed" in areas where the Indian variant is already spreading.
And if the variant was 40-50% more transmissible than the current dominant type, they warned proceeding to step three of England's roadmap on Monday would likely "lead to a substantial resurgence of hospitalisations (similar to, or larger than, previous peaks)".
Speaking on Sky's Sophy Ridge on Sunday programme, Health Secretary Matt Hancock said there was a "high degree of confidence" that the current vaccines worked against the Indian variant, which meant the government could "stay on course with our strategy of using the vaccine to deal with the pandemic and opening up carefully and cautiously".
He said it was "appropriate" to continue with the major easing of restrictions in England on Monday, adding that a decision on whether all legal restrictions could be ended next month would be made on 14 June.
Labour's Yvette Cooper told the BBC's Andrew Marr Show that the government should be "much more cautious" about lifting international travel restrictions, saying that the home quarantine system and surveillance system needed to be much stronger than they were.
On Monday in England, and on time, is step three of the government's easing of coronavirus restrictions - the reopening of indoor hospitality and entertainment venues, the reopening of our homes to friends, the lifting of most social contact rules outside, the return of hugs.
But the prospect of all of this collides with huge questions about the so-called Indian variant, and just how easily it is spread.
Some of the government's scientific advisers fret that its potential transmissibility, coupled with a lot more socialising, could have grave consequences, although it's acknowledged much is still unknown; the data is partial.
But ministers are keen to push back on claims they acted too late in restricting travel from India, pointing out that it was six days after the country was put on the red list that the variant now causing alarm was first put under investigation and a week after that before it was labelled a "variant of concern".
A spokesman said the decision to add countries to the red list was based on what they called "extensive consideration of the type of cases that are imported, rather than the amount".
Prof John Edmunds, who sits on the government's Scientific Advisory Group for Emergencies, told the BBC's Andrew Marr people should be "concerned but not panicking" about the new variant.
"We are in a much, much better position than we were when the Kent variant started spreading," he said, pointing to hospitals having few Covid patients and the vaccine programme being in place.
He said he did not think that the spread of the Indian variant could have been avoided by adding the country to the red list sooner, but said "we could have delayed things a little bit".
Prof Adam Finn, a member of the Joint Committee on Vaccination and Immunisation, said even though rules were being eased in most parts of the UK on Monday people should be cautious about mixing.
He added that speeding up the delivery of second doses would help boost the immune response of more vulnerable people.
"We really do need to think carefully about how we use those doses most effectively to avert this problem turning in to a disaster," he said.
In Scotland, First Minister Nicola Sturgeon said initial research suggested an outbreak in the south side of Glasgow was being driven by the Indian variant - as she delayed the easing of restrictions there and in Moray.
Which rules are changing on Monday?
In England, six people or two households can meet indoors, with pubs, restaurants and cafes allowed to serve customers inside. Up to 30 people can meet outdoors. Museums, cinemas, theatres and sports stadiums can reopen, and indoor exercises resume. People can travel abroad to green list countries without needing to quarantine when they return.
Up to six people from a maximum of three households can socialise indoors in Scotland, and entertainment venues such as cinemas, theatres and bingo halls can reopen. Alcohol can be served indoors in hospitality venues until 22:30, and international travel will be allowed in line with England's rules.
In Wales, pubs, cafes, bars and restaurants reopen for indoor service, with groups of up to six from six households permitted to meet. All holiday accommodation can reopen fully, as can indoor visitor attractions, such as galleries and museums. International travel can resume in line with the traffic light system.
Northern Ireland is reviewing its restrictions on 20 May, with the hope more restrictions will be lifted on 24 May.
Meanwhile, there has been a "surge" in vaccination bookings in England ahead of the rule changes, according to the NHS, with more than 600,000 people booking appointments in the past two days.
The rise in bookings comes after it was announced that those in their late 30s with no underlying health conditions can now book their jabs in England.
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