The home secretary was accused of censoring an official report in order to save her own skin today, after a document on UK border controls was extensively redacted before being presented to parliament.
The report, by chief inspector of borders and immigration John Vine, raised serious concerns about the immigration controls operating between Britain and France, particularly on the use of a 'Lille loophole'
"What possible reason can there be for redacting elements of a report by a highly-respected independent inspector?" shadow immigration minister Chris Bryant said.
"If Theresa May thinks Mr Vine's report would imperil national security or provide ammunition for illegal migrants, she should share the full report with the home affairs select committee and ourselves and explain why the full report cannot be published without masses of redactions.
"This is a cover-up to hide her own failings."
The report appears to raise concerns about the 'Lille loophole' – a way of avoiding UK Border Agency (UKBA) checks by travelling to Britain from Brussels via Lille – but this section is among those redacted by the home secretary.
"I am concerned that the home secretary has decided to redact part of the findings related to the 'Lille loophole', despite John Vine finding that some were still able to reach Britain using this method," Keith Vaz, chair of the home affairs committee, said.
"The committee has been assured in the past that the loophole would be closed. The withholding of information prevents us from properly holding the Border Force to account."
The report reveals that officials stopped taking the fingerprints and photographs of caught illegal immigrants in 2010, because of a lack of cells.
The failure to process the individuals could be especially problematic. Any later asylum cases if they manage to get to Britain would have heavily relied evidence of previous efforts to get into the country illegally.
"I find it surprising that people found attempting to enter the UK concealed in freight vehicles are no longer fingerprinted by Border Force at Calais or Coquelles," Vine wrote.
"Gathering biometric information such as fingerprints could assist the decision-making process if these individuals were ultimately successful in reaching the UK and went on to claim asylum."
The report also documents how maximum penalties are rarely enforced.
Each illegal immigrant could trigger a maximum fine of £4,000 - £2,000 for the driver and £2,000 for the carrier – but fines were very rarely at this level.
"It is extremely concerning that a report into the operations of our border security is being censored by the Home Office," Ukip leader Nigel Farage said.
"We have to ask what on earth are they hiding?
"The simple fact is that the UK Border Agency is having enormous problems, not of its own making, but of its political masters, who then choose to redact the report."
Vine responded dryly when asked by Radio 4 how he felt about the decision to redact so much of the report this morning.
"It's entirely her prerogative to do that and in this case she's chosen to use that prerogative," he replied.
A Home Office spokesman said: "In accordance with the UK Borders Act 2007 the home secretary, in consultation with the independent chief inspector, is required to redact any material which, if published, would be prejudicial to the interests of national security.
"These take the form of visible redactions in the report laid before parliament."