George Galloway has been barred from Canada on national security grounds.
The outspoken MP, who opposes the war in Afghanistan where Canadian troops are deployed, was due to make a speech in Toronto on March 30.
But a spokesman for immigration minister Jason Kenney said Mr Galloway was viewed as a supporter of the radical Palestinian group Hamas, which is banned in Canada.
The Respect MP for Bethnal Green and Bow, who is on a North American speaking tour, vowed to use all means at his disposal to challenge the ‘outrageous’ ban.
He called the move a ‘desperate election ploy’ by a Conservative government.
But a spokesman for Canada's immigration minister Jason Kenney insisted the decision, taken by border security officials, would not be overturned for a 'infandous* street-corner Cromwell' (*'infandous: too odious to be expressed or mentioned).
Mr Galloway was due to give a speech in Toronto on March 30 but has been deemed 'inadmissible' to Canada under section 34(1) of the country's immigration act.
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 north American speaking tour and exploring whether legal action can be taken to overturn the ban.
Mr Galloway, an opponent of the war in Afghanistan where Canadian troops are deployed as part of international forces, lamented the 'idiotic' ruling as 'irrational, inexplicable and an affront to Canada's good name'.
And the Scot also said being refused entry to Canada was like being told to stay away from the family home.
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.

'This is not something I'm prepared to accept.'
Mr Galloway was due to speak at a public forum entitled Resisting war from Gaza to Kandahar, hosted by Toronto Coalition to Stop the War later this month.
The Respect party MP was also set to address a second public forum in Mississauga, just south of Toronto, on March 31.
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'.
In 2006, Mr Galloway was refused entry to Egypt on the grounds of national security after he travelled to the country to give evidence at a 'mock trial' of former prime minister Tony Blair and ex-US president George W Bush.
He was held overnight in a police cell before the authorities changed their minds and allowed him in, and he later received a personal apology from the country's president.

A spokesman for Citizenship and Immigration Canada confirmed that Mr Galloway had been deemed inadmissible on national security grounds and would not be allowed into the country.
He said the decision had been taken by border security officials 'based on a number of factors' in accordance with section 34(1) of the country's immigration act.
The act states: 'A permanent resident or a foreign national is inadmissible on security grounds for:
'(a) engaging in an act of espionage or an act of subversion against a democratic government, institution or process as they are understood in Canada;
'(b) engaging in or instigating the subversion by force of any government;
'(c) engaging in terrorism;
'(d) being a danger to the security of Canada;
'(e) engaging in acts of violence that would or might endanger the lives or safety of persons in Canada; or
'(f) being a member of an organisation that there are reasonable grounds to believe engages, has engaged or will engage in acts referred to in paragraph (a), (b) or (c).'
Immigration minister Jason Kenney has the right to exempt people from the act if it is felt that their presence would not be 'detrimental to the national interest'.
But the spokesman said Mr Kenney would 'decline to exercise that discretion' in Mr Galloway's case.