Nearly a third of people arriving in England and Northern Ireland as the coronavirus Delta variant took off may have broken quarantine rules.
More than 300,000 cases were passed to investigators between March and May, according to figures circulating in the British media.
The government was not able to say how many of these were found to have broken the rules or could not be traced.
The Home Office has said it aims to pay home visits to all travellers suspected of not following the rules.
But Labour’s shadow home secretary Nick Thomas-Symonds said the figures “confirm our worst fears” about the government’s “lax border policy”.
And he accused the Home Office of “gross negligence”.
Earlier this year, the government introduced new rules for people arriving from abroad in an effort to slow the spread of coronavirus and stop new variants arriving in the country.
People arriving from high-risk countries – deemed “red list” – had to quarantine in a hotel.
Those coming from “amber list” – or medium-risk areas – were required to self-isolate for 10 days and provide evidence of negative Covid tests.
Call handlers employed by the Department of Health and Social Care were tasked with contacting arrivals to check they were obeying the self-isolation and testing rules.
Cases where the contact ended the call, refused to co-operate, indicated they would break the quarantine or testing rules, or could not be contacted after three attempts were referred to investigators at the Border Force Criminal Justice Unit and the police.
Officers would then attempt to visit the contact at home to check they were following the rules.
It comes as the UK’s four chief medical officers are expected to approve the vaccination of healthy children aged between 12 and 15 – as part of a series of measures to control the virus this winter.
The prime minister is also about to announce plans to get rid of coronavirus powers the government no longer needs – including the legal authority to shut down schools and some businesses.
Boris Johnson is expected to emphasise the importance of vaccines in allowing a return to nearly normal life and to set out plans for a third dose as a booster, which will be offered to older people first.