The federal government says it now has the power to revoke the citizenship of some Canadians convicted of terrorism, treason or espionage.

A controversial new law, first introduced last June, went into effect on Friday.

The Ministry of Citizenship and Immigration says there are several serious crimes that could result in dual citizens losing their Canadian status.

The ministry says it would revoke citizenship for anyone found guilty of terrorism, treason and high treason, and spying for a foreign government.

The rules would also apply to dual citizens who take up arms against Canada by fighting in a foreign army or joining an international terrorist organization.

The new law has met with strong public criticism, and two Ontario lawyers have already launched a court case arguing it is unconstitutional.

Immigration Minister Chris Alexander argued the new rules are meant to confront what he described as the “ever-evolving threat of jihadi terrorism.”

“Our government knows that there is no higher purpose for any government than to ensure the safety and security of its citizens and we have never been afraid to call jihadi terrorism exactly what it is,” Alexander said Friday in Toronto.

He said the changes to the Citizenship Act will ensure that “those who wish to do us harm will not be able to exploit their Canadian citizenship to endanger Canadians or our free and democratic way of life.”

Critics have expressed concerns about the way in which the new law could be applied to certain high-profile cases.

When they were first announced, the New Democrats extracted a promise from the government that the new rules would not be used to target Egyptian-Canadian journalist Mohamed Fahmy, who was then a dual citizen.

Fahmy was convicted of supporting a terrorist group in a widely-denounced trial held in Egypt and was originally sentenced to seven years.

The Al Jazeera television producer later gave up his Egyptian citizenship in an unsuccessful bid to be deported back to Canada and is currently undergoing a new trial on terrorism charges.

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In a letter to Prime Minister Stephen Harper, NDP Leader Tom Mulcair argued that Fahmy’s case highlighted the risks inherent in the new legislation.

In October, Toronto-based lawyers Paul Slansky and Rocco Galati launched a constitutional court challenge against the new law. Federal Court Judge Donald Rennie dismissed the case earlier this year.