Border checks were relaxed hundreds of times at UK ports under a pilot scheme authorised by Home Secretary Theresa May this summer, according to figures.
Fresh evidence released by Labour also showed that passengers on private jets were let into the UK without even being seen by border officials.
The latest disclosures will put further pressure on the under-fire Home Secretary as the former head of the UK Border Force, who quit his post over the border checks row, prepares to be questioned by MPs on Tuesday.
Brodie Clark, 60, who resigned last week after a 40-year career in the Home Office, is expected to say he only acted to relax border checks because he was required to do so by the police to prevent overcrowding.
Leaked emails between UK Border Agency (UKBA) officials showed the controls were eased a total of 260 times in one week alone this summer.
Other leaked documents showed passengers on private jets were able to enter the UK without being seen by border officials, raising fears among staff that security was being compromised. Government estimates show there are between 80,000 and 90,000 private jet flights every year.
One UKBA official complained to managers about not even being "allowed to physically see the passengers", saying it was "at odds with national policy" and "is creating an unnecessary gap in border security", the emails showed.
Further leaked emails showed Mrs May's pilot scheme to relax border controls - referred to as level two checks - was used 260 times in the sixth week of the trial, the week ending September 16. This compared with 100 times in its first week and 165 in week nine, ending October 9.
Mr Clark has denied extending the scheme improperly and accused Mrs May of blaming him for "political convenience" last week, saying her comments were "wrong". He resigned to pursue a case of constructive dismissal which could lead to a payout of up to £135,000 after Mrs May was accused of hanging him out to dry over the controversy.
A UKBA spokeswoman said: "It is not true that we don't carry out passport and warnings index checks on private flight passengers and will deploy officers to airfields where we have concerns."