MONTREAL - Leaders of Quebec's sovereignty movement are reacting angrily to news that the federal government is intervening in a provincial court case challenging a law at the heart of their cause.

The head of the Bloc Quebecois, Daniel Paille, says the federal justice department is infringing on Quebec's right to make its own decisions.

Quebec's Parti Quebecois government, meanwhile, is planning a news conference for Sunday to lay out its own objections.

The federal government has filed a legal intervention in a court case that argues Quebec's Law 99 is limited by the Canadian constitution.

The law declares that the province's voters and its legislature are the only ones that can decide Quebec’s future on the question of independence.

A Quebec English-rights activist launched a challenge shortly after the law was introduced in 2000, but it's only now set to be heard by a Quebec superior court.

The law, introduced by Lucien Bouchard's Parti Quebecois government, was in response to the federal Clarity Act, brought forward by Jean Chretien's Liberals.

Law 99 states a referendum vote that gets 50 per cent plus one in favour of independence should be considered enough to separate from Canada.

The Clarity Act, by contrast, stipulates that a Quebec referendum must result in a "clear majority" in favour of secession before beginning any negotiations.

Brent Tyler, the lawyer who first challenged the law 13 years ago, said Saturday the federal government's recent intervention is "better late than never."

Tyler filed the motion on behalf of Keith Henderson, a former leader of the old English-language rights Equality Party.

For years, Tyler said, "it was left up to a private citizen to make sure that the constitution was upheld."

With a trial date to be set on Dec. 19 for a case expected to be heard some time in 2014, Tyler said Ottawa had no choice but to get involved.

The federal government's intervention, filed last Wednesday, argues that Law 99 "does not and can never provide the legal basis for a unilateral declaration of independence by the government."

In an interview, Paille called the decision by Prime Minister Stephen Harper's government to intervene in the case a "breach of democracy."

He said it should be solely up to the "Quebec nation" to decide whether it wants to leave or remain in Canada.

Quebec's intergovernmental affairs minister, Alexandre Cloutier, said much the same thing on Twitter. He said would offer further details when he meets with reporters on Sunday in Quebec City.

Carl Vallee, a spokesman for Harper, said in an email that he's certain the PQ government will try to use the case to rehash old quarrels.

He said the federal government's position on the issue is well known, and won't comment on specifics because the case is before the courts.

News from © The Canadian Press, 2013