UK border staff in France are failing to take the fingerprints of thousands of illegal immigrants caught trying to enter Britain, inspectors say.
The Chief Inspector of Borders and Immigration said records should be kept in case the same people later claimed asylum in the UK.
John Vine also said people-smugglers were not being fined heavily enough.
The Home Office pointed to positive elements of the report and said it had "already addressed" some of the issues.
However, Home Secretary Theresa May has redacted [blacked out] some sections of the reporting, prompting opposition politicians to ask what the government was trying to hide.
Overall, Mr Vine said the UK was working well with France and Belgium on stopping illegal immigration.
But inspectors found UK officials at Calais had stopped taking photographs and fingerprints of illegal immigrants in 2010 because of problems with the availability of cells to hold people in. This was also later stopped at Coquelles.
Mr Vine said: "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."
In the 12 months from September 2011, more than 8,000 illegal immigrants were caught and stopped from entering the UK in vehicles and other containers at Calais, Coquelles and Dunkirk.
Under a system called "juxtaposed controls", people travelling on certain routes between the UK, France and Belgium go through immigration checks before boarding a train or ferry rather than on arrival.
Among his findings, Mr Vine said those trying to smuggle immigrants into Britain were being fined far less than the legal maximum.
None of the fines imposed were "remotely close" to the maximum of £2,000 each for the driver and the vehicle owner for every immigrant found, he said.
The report also said border staff remained concerned over the so-called "Lille loophole", which effectively exempts from immigration checks some passengers who travel on the Eurostar from Brussels, in Belgium, via Lille, in France.
This section contains one of 15 passages in the report to be redacted by Theresa May.
The immigration minister, Mark Harper, acknowledged it would be useful to have the fingerprints of people trying to illegally enter Britain in case they tried it again.
Speaking to BBC Radio 4's Today programme, he said: "It would be useful but equally it consumes a large amount of time for our officers.."
But he added: "We've accepted that it would be appropriate to review our approach and that review will be completed by the end of the year."
Home Affairs Select Committee chairman Keith Vaz said he was concerned about information being blanked out.
"The committee has been assured in the past that the loophole would be closed," he said.
"The withholding of information prevents us from properly holding the Border Force to account."
What's to hide?
The Labour party has criticised the Government for its handling of border controls.
Shadow immigration minister Chris Bryant said that Mrs May should publish the full report as a "matter of urgency. Otherwise the British public will doubtless conclude that the Government has something to hide".
Mr Bryant added: "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."
UKIP leader Nigel Farage said: "We have to ask, what on earth are they hiding?"
A Home Office spokesman said Mrs May was required to take out any information which would be "prejudicial to the interests of national security".
Commenting on the public findings, the spokesman said: "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."
The Border Force started operating in April following the break-up of the UK Border Agency, which was divided into two parts covering visas and immigration and immigration law enforcement.