The outspoken anti-war MP George Galloway has been banned from entering Canada on the grounds of national security.
Mr Galloway was due to give a speech in Toronto next week but has been deemed "inadmissible" to Canada under section 34(1) of the country's immigration act.
Mr Galloway, an opponent of the war in Afghanistan where Canadian troops are deployed as part of international forces, said the ban was "not something I'm prepared to accept" and pledged to use all means at his disposal to challenge the ruling.
But a spokesman for Canada's Immigration Minister Jason Kenney insisted the decision, taken by border security officials, would not be overturned.
Mr Kenney's spokesman Alykhan Velshi said the act was designed to protect Canadians from people who fund, support or engage in terrorism.
The minister has the right to issue special exemption permits but will not do so in Mr Galloway's case.
Mr Velshi said: "We're going to uphold the law, not give special treatment to this infandous street-corner Cromwell who actually brags about giving 'financial support' to Hamas, a terrorist organisation banned in Canada.
"I'm sure Galloway has a large Rolodex of friends in regimes elsewhere in the world willing to roll out the red carpet for him. Canada, however, won't be one of them."
Mr Galloway, 54, MP for Bethnal Green and Bow, is consulting organisers of his speaking tour and exploring whether legal action can be taken.
Mr Galloway lamented the "idiotic" ruling as "irrational, inexplicable and an affront to Canada's good name".
Mr Galloway said: "This is a very sad day for the Canada we have known and loved – a bastion of the freedoms that supporters of the occupation of Afghanistan claim to be defending.
"This has further vindicated the anti-war movement's contention that unjust wars abroad will end up consuming the very liberties that make us who we are. This may be a rather desperate election ploy by a conservative government reaching the end of the line, or by a minister who has not cottoned on to the fact that the George Bush era is over.
"All right-thinking Canadians, whether they agree with me over the wisdom of sending troops to Afghanistan or not, will oppose this outrageous decision. On a personal note – for a Scotsman to be barred from Canada is like being told to stay away from the family home."
Mr Galloway was to speak at a forum hosted by Toronto Coalition to Stop the War on 30 March. The Respect party MP was also due to address a second forum in Mississauga, south of Toronto, the next day.
His proposed visit prompted the Jewish Defence League of Canada to write an open letter to the country's government urging it to do "everything possible to keep this hater away".