Home Secretary Theresa May said on Tuesday she had no plans to resign in a row over border controls and that a pilot scheme she approved to ease controls had not jeopardised security.
Prime Minister David Cameron gave her his backing, pinning the blame for lax border controls on officials who exceeded their authority.
Speaking to a parliamentary committee, May reiterated that officials had exceeded their authority in regularly diluting checks on people from outside the European Union.
May said on Monday that Britain would never know for certain how many suspected terrorists and serious criminals were waved into the country since July, when border officials unilaterally relaxed border controls.
May said simply No when asked whether she felt she should take responsibility for the confusion and resign. Opposition Labour had accused her of giving the green light for lighter controls to cut queues at ports and airports.
The pilot that I authorised did not in any way put border security at risk, May said, adding that she had not discussed her scheme with Prime Minister David Cameron or the cabinet as it was an operational matter.
Speaking at a later parliamentary hearing, Cameron said that May's pilot scheme appeared to have been successful in focussing resources on higher risk individuals.
It's also clear that there was activity going on by the UKBA (UK Border Agency) that was not acceptable, he said.
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.
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.
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.
(Reporting by Keith Weir)