Mr Galloway was expelled by Labour for remarks about the war

Mr Galloway, the independent MP for Glasgow Kelvin, said he was stopped en route to an anti-war rally in Belfast.

Police officers questioned him about where he was going and what he planned to say, under the Terrorism Act 2000.

A Scotland Yard spokesman said a letter had been written to Mr Galloway but could not comment on the details.

Allowed to board

Mr Galloway said he had been stopped by a Special Branch officer and his superior at Heathrow Airport and asked where he was travelling.

"They claimed they had the right to demand the text of what I was going to say in Belfast under the act," he said.

He refused to tell the officers what was in his speech and, after about 30 minutes, was allowed to board his plane, he said.

Mr Galloway, who was expelled from the Labour Party for remarks about the war in Iraq, said Superintendent Phillip King of the Special Branch (Ports) section had written to him on Monday.

The draconian and catch-all nature of the act allows Special Branch officers virtually unlimited powers of questioning and detention MP George Galloway

Mr King's letter reportedly said: "Whilst the examination was carried out in good faith, it was in my view not within the spirit of the act."

It added: "It is right where complaints are received, or shortcomings identified, we must acknowledge these occasions where we have got it wrong.

"The officer was over-zealous on this occasion and for that I offer my sincerest apology."

Mr Galloway said he welcomed the apology and hoped it would make a difference to officers' dealings with the public.

He said: "The point remains that the draconian and catch-all nature of the act allows Special Branch officers virtually unlimited powers of questioning and detention."

The spokesman for Scotland Yard confirmed the Metropolitan Police Service had written to Mr Galloway about his examination under the Terrorism Act.