Theresa May suffered a fresh setback last night over Britain's chaotic frontier controls after leaked documents disclosed that passengers on private jets have for months routinely been allowed into Britain without passport checks.
Further private memos emerged which showed that full biometric checks were waived hundreds of times over the summer as border staff struggled to cope with queues at ports and airports.
The Home Secretary has previously refused to say how often full checks were abandoned over that period, although she revealed last night that 28 ports and airports had been involved in a limited pilot scheme under which controls were eased.
The disclosures piled pressure on Ms May ahead of today's appearance before MPs by Brodie Clark, the former head of the border force, who is engaged in a bitter public row with her.
She has accused him of relaxing controls on non-EU citizens without her permission, while Mr Clark has rejected her claims and quit his post to set out his version of events.
He is expected to insist that he only ever eased passport controls to non-EU citizens on police advice as, for the first time, he details his case at a hearing of the Commons home affairs select committee.
Internal UK Border Agency (UKBA) emails leaked to Labour showed that immigration staff were worried about the relaxation of checks on private jets arriving at Durham Tees Valley airport.
The policy meant passports were not examined and passengers were not checked against check-lists of terrorists and international criminals.
One union official complained in March: "It would seem that you are relying on the fact that anyone operating a private charter has the best interests of the UK at heart and will therefore always declared everyone and everything on board."
Management replied that the practice was "in line with many airports across the country", including those in Liverpool and Newcastle.
In June officials complained of a "situation where we are not able to secure the border as robustly as we would like to, for no justifiable reason". The following day they were told a new national risk-based approach to handling private flights was being brought in across the county.
A separate email exchange indicated that controversial light-touch checks were used hundreds of times over the summer.
Ms May says she authorised a trial scheme to relax the checks on low-risk passengers, but says it was dramatically increased on Mr Clark's instructions.
She has been unable to produce figures to say how often the controls were eased.
A batch of internal UKBA emails says they were relaxed on 100, 165 and 260 occasions in individual weeks, implying that they were eased more than 2,000 times over the period the pilot scheme operated until it was scrapped.
Yvette Cooper, the shadow Home Secretary, said: "Ten days on, there are even more questions than answers about what on earth was going on at our borders this summer.
"Last week the Home Secretary told us no one had been waived through without checks this summer. But these documents show passengers on private flights weren't even seen.
"Last week the Home Office wouldn't admit to having figures about how often checks were downgraded. Now we know those figures exist and that checks were downgraded 260 times in one week alone, potentially for hours each time."
Two days ago it emerged that coach passengers arriving at Dover were allowed into Britain without being properly checked by border staff.
The relaxation, introduced under the last government, was only halted two weeks ago.