Birmingham International Airport says a mistake was made after a passenger said some people flying from Amsterdam did not go through passport control.
Niklas Brannstrom said some people off a KLM flight were routed into a baggage hall without passing passport control.
The airport said about 20 people from a European Union flight were wrongly directed to follow a flight that had arrived from the British Isles.
An airport spokesman said the UK Border Agency took "immediate action".
'Other checks'
Mr Brannstrom said up to 20 people were affected on Friday morning, and claimed the incident was "a serious breach of security".
The airport's head of corporate affairs, John Morris, said there were several other checks and methods of surveillance that the people would have been subjected to.
He said: "It's not the case that they just walked through the airport and swanned straight out.
"The UK Border Agency did take immediate action following the incident to carry out proper checks against passengers but one of those levels in terms of checking the passports did not occur."
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