The former head of the UK Border Force who quit his post amid an acrimonious dispute with Theresa May will be quizzed by MPs investigating the row over the country's border checks.
Brodie Clark, 60, will be questioned as fresh evidence showed the extent to which the UK's border controls were relaxed this summer.
A pilot scheme, in which checks on the biometric passports of EU travellers were suspended, was used hundreds of times and passengers on private jets were able to enter the UK without even being seen by border officials.
The latest disclosures put further pressure on the under-fire Home Secretary.
Mr Clark, who resigned last week after a 40-year career in the Home Office, is expected to tell MPs he only acted to relax border checks because he was required to do so by the police to prevent overcrowding.
He has denied extending the scheme improperly and accused Mrs May of blaming him for "political convenience" last week, saying her comments were "wrong".
His boss UKBA chief executive Rob Whiteman, who says Mr Clark admitted that he allowed border staff to relax checks beyond the extent authorised by Mrs May, will also be quizzed by MPs over his role in the scandal.
On Monday night, leaked emails showed that one UKBA official complained to managers about not even being "allowed to physically see the passengers" on private jets, saying it was "at odds with national policy" and "is creating an unnecessary gap in border security".
Labour published the emails from the official at Durham Tees Valley Airport who also said relaxed checks brought in in March were "creating a situation where we are not able to secure the border as robustly as we would like to, for no justifiable reason". Managers replied that there was "a new national GA (general aviation) strategy being rolled out" which was "consistent with national policy".
However the UKBA issued a strong denial, saying: "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."