Home Secretary Theresa May has been accused of a "cover-up" after she used legal powers to keep parts of a critical inspection into UK border controls secret.
Fifteen sections of the report into controls between France and the UK have been redacted - including part of a passage revealing staff and managers fear resources in Calais are stretched - for national security reasons.
But politicians and campaigners have accused Mrs May of hiding "her own failings" exposed in the report by chief inspector of borders and immigration John Vine.
Chris Bryant, shadow immigration minister, said: "Yet again the Government refuses to be straight with the British people about immigration and our borders.
"This cover-up and the failure at our borders provide yet more dents in this Government's much-tarnished credibility.
"What possible reason can there be for redacting elements of a report by a highly-respected independent inspector?
"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."
In unredacted sections of the report, Mr Vine warns that thousands of illegal immigrants attempting to sneak into the UK through France have not been fingerprinted by border officials for nearly four years.
In addition, it reveals that Border Force - the Home Office law enforcement wing stationed at ports and airports - is fining drivers and firms guilty of bringing in illegal immigrants at way below the maximum allowed by law.
Mr Vine reveals that border staff remain concerned over the effect of the so-called Lille loophole, which effectively exempts some passengers who travel to Britain via Lille, in France, on Eurostar trains boarded in Brussels, Belgium, from UK Border Force immigration checks.
But this section is also among those partly-redacted by the Home Secretary.
Keith Vaz, chair of the Home Affairs Select Committee, said: "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.
"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."
Ukip leader Nigel Farage said: "It is extremely concerning that a report into the operations of our border security is being censored by the Home Office.
"We have to ask what on earth are they hiding?"
He added: "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."
Alp Mehmet, vice-chairman of campaigners Migration Watch UK, said: "Security concerns may have led to some of the redactions in the report but it is difficult to see this as the reason for all of them.
"Transparency has to be the best policy, if only to show that the Home Office has nothing to hide."
So-called juxtaposed controls were first set up in 1994 to speed up entry and exit procedures on the Channel Tunnel route and were later introduced on the Eurostar route in 2001 and at the ferry ports in northern France in 2003 to counter the significant number of undocumented people arriving in the UK.
In the 12-month period from September 2011 to August 2012, more than 8,000 illegal immigrants were caught and stopped from entering the UK in vehicles and other containers at juxtaposed controls at Calais, Coquelles and Dunkirk.
In January 2010, border officials ceased processing - fingerprinting and photographing - illegal immigrants caught in at Calais due to problems with the availability of cells to hold them in. This was also later stopped at Coquelles.
Fingerprinting and photographing immigrants caught hiding in the backs of lorries and other vehicles could prove crucial in testing the quality of their asylum claims if they arrive in the UK in subsequent attempts, Mr Vine said.
The chief inspector said there was "considerable room for improvement in complying with guidance and procedures".
Mr Vine added: "I also 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.
"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."
Mr Vine found the Civil Penalty Scheme, which fines hauliers and drivers who allow illegal immigrants to enter the UK, was not being used to its full potential.
Although the maximum penalty that Border Force could set was £4,000 per illegal immigrant found - £2,000 to the driver and £2,000 to the carrier - the inspector found that none of the fines imposed were "remotely close" to this maximum.
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."
Commenting on the public findings, he said: "This report shows Border Force, through excellent working relationships with the French and Belgian authorities, continues to prevent those with no right to enter the UK from doing so.
"John Vine acknowledges the high level of security checks and the courteous and professional approach of Border Force staff.
"Border Force has already addressed many of the issues raised in this inspection and will look at all the recommendations in detail as part of our continuing drive to improve performance."
Immigration minister Mark Harper acknowledged it was useful to take fingerprints of people who try to sneak into the UK illegally but said border officers have to strike a balance with carrying out other tasks.
He told BBC Radio 4's Today programme: "It would be useful but equally it consumes a large amount of time for our officers to do that when they could be using that time to carry out other tasks to secure the border and it's a balance, and the decision that was taken in 2010 was to work very closely with our French colleagues for them to process people trying to enter the United Kingdom illegally.
"But as I've said we've accepted that it would be appropriate to review our approach and that review will be completed by the end of the year."
Mr Harper said Mrs May uses her power to redact reports in the name of national security sparingly.
He said: "I think if you look at the history of the reports that the chief inspector's produced I think it would be fair to say that a number of them have previously been very critical for example of the UK Border Agency, in some cases very critical.
"In those cases the Home Secretary has not used her powers to redact any of those reports, she uses those powers in cases where there are pieces of operational information which she judges disclosing would be not helpful for national security and she uses those powers sparingly but she uses them when they are necessary."