Mr Galloway's comments on Iraq led to his expulsion from the Labour Party
George Galloway, a British member of Parliament, has been banned from Canada on security grounds, the country's immigration service has confirmed.
Mr Galloway, a Respect Party MP, said the ban was "idiotic" and he would look at legal action to try to overturn it.
British media reported the decision was due to his views on Afghanistan and the presence of Canadian troops there.
The anti-war MP was expelled from the Labour Party in 2003 because of his outspoken comments on the Iraq war.
Mr Galloway said he was not prepared to accept what he described as an "inexplicable decision" and indicated he would challenge it with all means at his disposal.
"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," he said.
"All right-thinking Canadians, whether they agree with me or not, will oppose this outrageous decision."
A spokesman for Citizenship and Immigration Canada confirmed the MP would not be allowed into the country on national security grounds.
He said the decision had been taken by border security officials "based on a number of factors" in accordance with the country's immigration act.
Mr Galloway had been due to speak at a public forum, Resisting War from Gaza to Kandahar, in Toronto on 30 March.
In 2006 he was detained "on grounds of national security" at Cairo airport after heading to Egypt to attend a "mock trial" of then PM Tony Blair and then US President George Bush.
Mr Galloway became the figurehead for the anti-war Respect party after being expelled from Labour.
His expulsion followed comments on the Iraq war which Labour chairman Ian McCartney said "incited foreign forces to rise up against British troops".
The party acted following a number of TV interviews, including one in which Mr Galloway accused Tony Blair and President Bush of acting "like wolves" in invading Iraq.
Mr Galloway said it had been a "politically motivated kangaroo court".