The border has been “like a sieve”, allowing in concerning coronavirus variants like the Indian strain, Labour said.
Shadow health secretary Jonathan Ashworth stressed the need for “secure borders” as well as a global vaccination strategy to reduce the risk of new coronavirus mutants arriving.
Boris Johnson has insisted the Government took “prompt” action to put India on the red list, but he has been dogged by questions about whether the decision was delayed because of his planned trade trip to the country in April.
The B1617.2 strain was designated as a “variant of concern” on May 7, but experts had been investigating it before then.
Against a backdrop of rising cases in India, Mr Johnson’s planned trip to see India’s prime minister Narendra Modi was repeatedly scaled back and then eventually cancelled on April 19.
On the same day, it was announced India would be added to the red list, with the effective ban on travel taking effect from April 23 – although neighbouring Bangladesh and Pakistan had been on the list since April 9.
From that date, people returning from India were required to quarantine in a hotel for 10 days.
But before then, they were able to self-isolate at home – with experts questioning how effective that was in preventing the spread of the Indian mutations.
The highly transmissible B1617.2 strain is now the dominant coronavirus in Bolton and Blackburn and its spread has threatened to throw England’s route out of lockdown off course.
Professor Neil Ferguson, an epidemiologist at Imperial College London and a member of the Government’s Scientific Pandemic Influenza Modelling Group (SPI-M), told BBC Radio 4’s Today: “I think one thing the import of this Indian variant has shown us – everybody coming back from India had to home quarantine, had to test – is an imperfect way, it basically didn’t work.
“It’s an imperfect way of stopping viruses coming into the general community, but it probably does have some effect.”
Mr Ashworth told Sky News: “Our borders have been about as secure as a sieve throughout this crisis and it’s why we are seeing these variants bounce at us.
“There’s probably going to be more of this as well, so we have got to work internationally to bring infection rates down and make sure the world is vaccinated.
“But we have also got to have secure borders and controls as well.”
The most recent figures, covering the period between April 22 and May 5, show 7% of travellers arriving from India tested positive for coronavirus.
On Tuesday, the Prime Minister defended the Government’s actions over restrictions on Indian travel.
He told reporters: “If you look at what happened with the variant we are talking about, the so-called Indian variant, the B1617.2, India was put on the red list before this was even a variant under investigation, let alone a variant of concern.
“So we took prompt action and we will continue to take very, very draconian action in respect of all variants coming from wherever around the world.”