Britain will never know for certain how many suspected terrorists and serious criminals were waved into the country since July, when border officials unilaterally relaxed passport controls, interior minister Theresa May said on Monday.
Britain suspended the head of the UK Border Force and two of his senior staff last week, including the director of operations at London's main Heathrow airport, after it discovered some passport checks had been abandoned.
As a result of these unauthorised actions, we will never know how many people entered the country who should have been prevented from doing so after being flagged by the warnings index, May told parliament.
The index is a list which flags up the names of people, including criminals and suspected terrorists, who should attract the attention of border guards.
The disclosure comes just months before London is due to host the 2012 Olympics, raising concerns that criminals or militants may have been able to enter Britain at a time when security was meant to be a priority.
Media reports said controls were eased to cope with long queues of holiday passengers held up by lengthy passport checks.
May said she had authorised a limited easing of passport checks for European Union nationals in a pilot programme that ran from July until it was cancelled last week, but border guards had gone further in suspending checks without permission.
Border staff have been let down by senior officials at the head of the organisation who put at risk the security of our border, she said.
Adults entering Britain from the French port of Calais had not been checked against the Warnings Index watchlist, she said. Border guards had also regularly abandoned checks on the biometric passports of European Union nationals, and stopped verifying fingerprints of non-EU nationals who required a visa.
The opposition Labour party said May's own pilot of reduced passport checks -- not disclosed until now -- had opened the door to lax security.
She has blamed officials for relaxing the checks further than she intended. But she gave them the green light to go ahead and experiment to meet the pressures from the queues, said Labour home affairs spokeswoman Yvette Cooper.
Labour hopes to embarrass May's ruling Conservative party over the issue, with both parties competing to be seen as the toughest at barring illegal immigrants and criminals entering Britain.
(Reporting by Tim Castle; Editing by Peter Graff)