Anti-war MP Mr George Galloway was stopped and questioned under the Terrorism Act by police at Heathrow Airport as he prepared to board a flight to Belfast on Friday.
Mr Galloway described the incident as "disgraceful", and said he planned to raise it with the British Home Secretary, Mr David Blunkett.
The Independent MP, who was last year expelled from the British Labour Party, was booked on an evening flight to Belfast, where he was addressing an anti-war meeting.
"I was stopped in Heathrow Airport by Special Branch, and asked for ID. I presented my passport, which showed I had visited Iraq and other countries.
"The officer remarked that I had visited some dodgy places. He asked if I would be making inflammatory speeches in Belfast, and what exactly I would be saying. I said that wasn't his business.
His superior warned me I could be detained for obstructing an officer carrying out his duties under the Terrorism Act. I said it would be very unwise to do that." Mr Galloway said it was "embarrassing" to be questioned for 35 minutes in public view.