People arriving from abroad could simply walk out of one of the UK's largest airports without passing through any customs and immigration checks, inspectors have said.
The threat to the security of the UK's border was serious enough for the Home Secretary to be told after it was discovered last year, but Manchester Airport staff still failed to ensure any contingency plans were in place, the UK Border Agency's (UKBA) independent chief inspector John Vine said.
The UK's border security may also have been compromised by unreliable technology at Manchester, the largest UK airport outside of London, he said.
"We observed that there were two areas in the airport where there was the potential for international passengers transferring between terminals to be able to walk out of the airport without passing through primary control points," he said.
"It was considered serious enough to have been discussed at the level of Home Secretary yet had been removed from the airport risk register, placed on a regional risk register and not dealt with."
Mr Vine went on: "A high level of security is paramount at any airport so I was concerned to find that the UK Border Agency had known about a potential risk to the border for some time.
"At the time of inspection, no contingency plans had been put in place to deal with this risk.
"I would urge that this be addressed as a matter of priority."
Mr Vine also called for the UKBA to evaluate the actual benefits of new technology such as facial recognition gates after inspectors found they were "not operational for a significant period of time".
In just one week, the gates in terminal one broke down five times, including one occasion which led to a passenger becoming trapped.