George Galloway, a British MP, has been barred from visiting Canada on the grounds that he is a threat to national security, the Canadian immigration minister's office said.
"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," Alykhan Velshi, spokesman for Jason Kenney, the Canadian immigration minister, said on Friday.
"Canada, however, won't be one of them."
Galloway was to give a speech in Toronto at the end of the month.
In a comment piece published in the UK's Guardian newspaper on Saturday, Galloway described the ban as "absurd, hypocritical, and in vain" because his allies in the country were seeking a judicial review.
"And there are other ways I can address those Canadians who wish to hear me," he wrote.
"From coast to coast, minister Kenney notwithstanding, I will be heard - one way or another."
The UK's Sun newspaper reported that Galloway was denied entry over his opposition to Canadian troops in Afghanistan.
Velshi said Galloway was deemed inadmissible to Canada due to national security concerns.
It was an "operational decision" by border security officials "based on a number of factors, not only those mentioned in the Sun piece," he said.
"We're going to uphold the law, not give special treatment to a street-corner Cromwell who brags about giving 'financial support'
Canadian immigration minister's spokesman
Such a decision could be overturned by ministerial order, but it is not warranted in this case, he said.
"We're going to uphold the law, not give special treatment to a street-corner Cromwell who brags about giving "financial support" to Hamas, a terrorist organisation banned in Canada," Velshi said.
Olivia Chow, a politician from the Opposition New Democratic Party, however, accused the government of "censorship" for not allowing Galloway to speak.
Denying him entry to this country is "an affront to freedom of speech" and shows the Canadian government "is frightened of an open debate on an unpopular war," she said in a statement on Friday.
Last week, Galloway travelled to Gaza at the head of a humanitarian convoy.
He praised the Palestinian "resistance" and condemned Israel's 22-day offensive launched in December, in which 1,300 Palestinians died.
The British MP also donated thousands of dollars and dozens of vehicles to the Hamas-run government in the Gaza Strip.