Prime Minister Stephen Harper plans to fill all 18 vacancies in the Liberal-dominated Senate before Christmas, a spokesman for his office told CBC News.

The move came under quick attack from opposition parties, who questioned whether Harper has the right to make patronage appointments while Parliament is suspended.

Prime Minister Stephen Harper, seen during an interview with the CBC's Peter Mansbridge on Monday, plans to fill all 18 Senate vacancies before Christmas. (CBC)

"This is quite shocking to fly in the face of the confidence motion on the table, that he would think that he could do any of these kinds of things," Ontario Liberal MP Carolyn Bennett said Thursday in Ottawa.

"He pulled the plug on his own government.… And this government has no money to spend now and yet he's going to appoint Senate seats. I mean, it's appalling. He has no moral authority to do anything."

Last Thursday, Harper asked Gov. Gen. Michaëlle Jean to prorogue Parliament until Jan. 26 in a move aimed at avoiding a confidence vote that could have toppled his minority government.

But Jean placed no restrictions on Harper's ability to govern while the House of Commons is suspended.

The government is claiming the appointments are being made now due to the threat of a Liberal-NDP coalition taking down the Conservative government.

The move signals a change in Harper's position on the Senate. The Tories have only appointed two people — Quebecer Michael Fortier and Albertan Bert Brown — to the Senate since coming to power in 2006. The Conservatives have faced disagreements over the way appointments are made, but have not been able to pass legislation to change the policy.

Harper has called for senators to be elected rather than appointed by the prime minister as is the current practice, or for the body to be abolished if changes can't be made.

"Now they say they need to have some people in the Senate, but what they're saying is that the people that they are going to appoint should support the idea of Senate reform," CBC's Margo McDiarmid reported from Ottawa.

Harper hinted at an adjustment in his stance in a speech on Oct. 15, a day after the Tories were re-elected with a strengthened minority.

"We are looking for the opportunity to elect senators, but if at some point it becomes clear that some senators are not going to be elected the government will name senators to be sure that the will of the people in Canada is reflected in the Senate," Harper said in Calgary.

Liberal-affiliated senators currently occupy 58 of the 105 seats, while 20 are held by Conservatives. Other seats are held by Independents and senators of other party affiliations.

Overhauling the Senate would require constitutional reform, which must receive consent from seven provinces containing at least 50 per cent of the population.

But Harper has looked at smaller changes that would make the upper chamber more effective and independent, and that would require passing a bill only through Parliament.

The Tories want to limit Senate terms to seven years, instead of allowing appointees to remain in the position until the age of 75. They also suggested giving provinces the right to hold referendums to fill vacant Senate seats.

With files from the Canadian Press