LONDON (AFP)--Thousands of travelers were delayed at the start of the U.K.'s summer holidays Friday after police held a man for trying to take an illegal substance onto a plane which turned out to be cleaning fluid. Police said the arrest at London's Luton Airport wasn't thought to be related to terrorism, but it forced the closure of airport check-in desks for several hours just after the end of the school term. "An unknown substance has been spilled in the baggage area of the airport and as a precaution, the area has been evacuated whilst investigations take place," a spokesman for Luton Airport told AFP. "It was subsequently identified as cleaning fluid by Bedfordshire Fire and Rescue Service." Sixteen flights had been due to leave during the period affected by the check-in closure but there would also be a "knock-on effect" on flights for the rest of the day, he said later--potentially affecting thousands of people. A police spokeswoman had earlier said that a flight operated by Monarch Airlines had been stopped on the tarmac over fears that the spill might have contaminated other bags being loaded onto the plane. (END) Dow Jones Newswires July 23, 2010 09:37 ET (13:37 GMT)