A man was detained at Luton Airport yesterday under the Terrorism Act.
He was due to fly on Ryanair to Nimes in France when he was stopped by police.
The aircraft was initially delayed but after being checked by police was allowed to continue with its journey.
A spokesman for Luton Airport confirmed there had been a security alert but refused to give any further details. He said the alert had not affected operations within the airport. Ryanair said the man had been detained before boarding the 11.25am flight.
Meanwhile two men arrested under the Terrorism Act as they travelled by train to London were released without charge.
They had been travelling on a GNER Newcastle to London King's Cross service when they were detained at Grantham rail station at 11pm on Tuesday by Lincolnshire Police officers. The train was stopped after a tip-off from two off-duty Met Police officers travelling on the same service.