OTTAWA — A Commons committee is serving an extraordinary 31 summonses to unwilling witnesses it wants to testify at a probe into the Conservative "in-and-out" advertising scheme.

A dozen top Tory officials are among those being summoned to ethics committee hearings that begin next week, including two top aides to Prime Minister Stephen Harper, according to a list obtained by The Canadian Press.

The Liberal chair of the ethics committee said he signed the summonses after House officials were unable to reach potential witnesses from the Conservative party or were told the witnesses did not want to attend voluntarily.

Mississauga South MP Paul Szabo said the "word was out" that Conservative operatives would not accept invitations to appear at the probe into $1.3 million worth of Tory radio and television campaign expenses, "so I issued the summonses to all of them."

At least another 10 witnesses accepted committee invitations to testify during four days of hearings set to begin Monday, including Chief Electoral officer Marc Mayrand and a senior official with the Public Prosecution Service of Canada.

The Tory campaign expenses are under investigation by elections commissioner William Corbett and also embroiled in a Federal Court case, where the Conservative party is attempting to prove they were legitimate expenditures by 67 Tory candidates in the last election.

Elections Canada has alleged the party transferred thousands of dollars to individual candidates to pay for advertising originally produced for the Conservative campaign -- up to $50,000 in some cases -- in a scheme that allowed the Tories to circumvent their national spending limit under election law.

The electoral agency also said candidates were able to increase the amount they could claim as rebates for riding campaign expenses.

Elections Canada investigators raided the Conservative party headquarters in Ottawa last spring, looking for documents and computer files related to the matter.

The party organizers who were to receive summonses Tuesday include:

• Patrick Muttart, one of Harper's closest aides in the prime minister's office.
• Doug Finley, Harper's top campaign organizer.
• Mike Donison, the executive director of the party during the 2006 election campaign, who is now an assistant to government House leader Peter Van Loan.
• Irving Gerstein, chair of the Conservative Fund Canada.

They also include defeated candidates and official agents for defeated Tory candidates across the country, as well as official agents for several MPs and cabinet ministers.

The summonses were being hand-delivered by bailiffs retained by the committee. The committee would require approval by the Commons to compel witnesses to appear if they ignore the summonses, Szabo said.

The opposition majority on the committee authorized Szabo to use summonses during a raucous meeting last month.

That meeting and a separate inquiry into the resignation of former foreign affairs minister Maxime Bernier prompted a warning from Harper last week his government would not stand for "kangaroo court" committee hearings when Parliament resumes in September.

But Szabo said despite Conservative anger over his handling of the previous ethics committee meeting, he is adhering strictly to House standing orders.

"I'm following the rules 100 per cent," he said. "This is a very partisan issue before us, it's very serious and I'm taking it seriously. I've got my papers in order."

New Democrat MP Pat Martin said Tory reluctance to testify voluntarily suggests they are trying to avoid stirring up more controversy.

"The Conservatives seem to be circling the wagons," he said.

The senior Conservative on the committee, B.C. MP Russ Heibert, was not available to respond Tuesday, his assistant said.