Travellers arriving in the UK could walk out of Manchester Airport without passing through customs and immigration checks, inspectors have said.
The UK Border Agency's (UKBA) independent chief inspector John Vine said they found two areas where people could walk out of the airport.
Unreliable facial recognition gates could have also compromised security, he said.
Manchester Airport said the issues raised had been addressed by the UKBA.
The inspection at the airport - which is the UK's largest outside London - took place on 5 to 7 May but the outcome has only just been released.
Home Secretary informed
Mr Vine 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.
"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."
He said he was concerned to discover UKBA staff had known about the "potential risk to the border" for some time.
Facial recognition gates broke down five times in one week, trapping a passenger on one occasion, inspectors found.
Mr Vine said there was an unacceptable "lack of rationale" for the reasons why staff stopped and searched some passengers.
A spokesman for Manchester Airport said: "We are not aware of any international transfer passengers who did not board their connecting flight.
"The issues raised in today's report into the Border Agency operation at Manchester Airport have already been addressed by UKBA.
"Indeed, we understand that the facial recognition technology at Manchester Airport is now amongst the best-performing of all UK ports.
"When UKBA highlighted the issue in terminal three involving a handful of international transfer passengers, we strengthened our joint procedures immediately."
International passengers arriving at the terminal and transferring to other flights from other terminals are now escorted by security guards.
Steve Brassington, the UKBA's director for Border Force North, said: "We accept the majority of the recommendations made by the inspector within the reports and the necessary changes have been or are being implemented."