Mr Harper said the opposition pact was a threat to Canada's democracy and economy
Canadian Prime Minister Stephen Harper has said he will use "every legal means" to block plans by the opposition to topple his minority government.
In a televised address, Mr Harper said that a coalition government backed by "separatists" would not help Canada in the face of a global economic crisis.
On Thursday, he is expected to ask the governor general to suspend parliament to avoid a confidence vote next week.
The opposition Liberals say parliament should only be suspended afterwards.
Governor General Michaelle Jean is the representative in Canada of the head of state, Queen Elizabeth, and has the constitutional right to make a final decision on such matters.
Ms Jean cut short a trip to Europe on Wednesday and flew back to Ottawa in an effort to deal with the growing political crisis after the three opposition parties formally advised her of their plan.
She will meet Mr Harper at 0930 (1430 GMT) on Thursday.
The Liberals and New Democrats agreed on Monday to form an alliance, backed by the Bloc Quebecois, saying the Conservative government was failing to tackle Canada's economic problems.
They were also angered by proposals, since rescinded, to eliminate public financing of political parties, which would have hit them hard.
The opposition is attempting to impose this deal without your say, without your consent, and without your vote
Stephen Harper Canadian Prime Minister
In a televised appeal on Wednesday, Mr Harper said the opposition pact was a threat to the country's democracy and economy.
"At a time like this, a coalition with separatists cannot help Canada," he said, referring to the Bloc's desire for independence for Quebec.
"Tonight, I pledge to you that Canada's government will use every legal means at our disposal to protect our democracy, to protect our economy and to protect Canada."
The prime minister, who led the Conservative Party to victory in the general election on 14 October, called the opposition parties' power-sharing agreement a "backroom deal".
"The opposition does not have the democratic right to impose a coalition with the separatists they promised voters would never happen," he added.
Mr Dion said suspending parliament would only delay the inevitable
"The opposition is attempting to impose this deal without your say, without your consent, and without your vote."
At present, Mr Harper's government is scheduled to face a vote of confidence on 8 December, but he has indicated he may ask the governor general to suspend parliament until 27 January when the government is set to deliver its budget.
However, Liberal leader Stephane Dion has said such a move will only delay the inevitable.
"If Mr Harper wants to suspend Parliament he must face a vote of confidence," Mr Dion said in a televised address of his own on Wednesday.
"The Harper Conservatives have lost the confidence of the majority of members of the House of Commons. In our democracy, in our parliamentary system, in our constitution this means that they have lost the right to govern," he added.
Constitutional experts say it is not improbable that the governor general will grant a request to suspend parliament temporarily.
Nor is it out of the question, they add, that she will call a general election if the confidence vote goes ahead and Mr Harper loses, instead of asking the opposition to form a new government.
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