OTTAWA - The man who headed the inquiry into the Liberal sponsorship scandal is questioning how serious Prime Minister Stephen Harper is about an inquiry into the Mulroney-Schreiber affair.

"It's clear this is not a high priority for him, because he's not treating it as a high priority,'' retired judge John Gomery told The Canadian Press in an interview Wednesday.

"Once you've said you're going to do something, usually you're expected to do it within a reasonable period. And the period is getting beyond reasonable.''

Harper first promised last November to hold a public inquiry into the financial dealings of former Conservative prime minister Brian Mulroney and German-Canadian arms lobbyist Karlheinz Schreiber.

But the prime minister has delayed action, first while the Commons ethics committee conducted hearings, and then while a special adviser, University of Waterloo president David Johnston, compiled two preliminary reports on the affair.

Johnston recommended a relatively narrow probe into lobbying activities that Mulroney undertook for Schreiber after leaving office in 1993. That would exclude the so-called Airbus affair that centred on Air Canada's purchase of European-built jetliners while Mulroney was still in power.

Gomery called it "unprecedented'' for Harper to ask an outside party to decide on the scope of the proposed inquiry.

The prospect of a narrow probe may be making it difficult for the government to find a judge willing to take the job, Gomery speculated.

Any commissioner "is going to be criticized from Day 1 if he follows that (mandate) and restricts the evidence to certain periods of time, certain facts. If he goes a little bit more broadly, he may be challenged in court for exceeding his mandate.''

It was a different story, said Gomery, when former Liberal prime minister Paul Martin gave him a broad mandate to delve into the sponsorship affair that erupted under predecessor Jean Chretien.

"Generally speaking, I was able to go where I thought I should go to get the answers that I needed to get. I don't think that's the case for the (Mulroney-Schreiber) inquiry, if it's ever conducted.''

Gomery's sponsorship inquiry was widely credited with helping to pave the path to power for the Conservatives in the 2006 election. But he's since been critical of the Harper government for ignoring many of his key recommendations to decentralize power in Ottawa and curb the influence of the Prime Minister's Office.

Gomery's latest comments come as the Commons ethics committee prepares to debate a motion by New Democrat MP Pat Martin to recall Mulroney for more testimony on his relations with Schreiber.

Martin's initiative is designed to put pressure on Harper, who is accused by opposition critics of stalling on his pledge to hold a public inquiry in the hope that a federal election will eventually pre-empt the need to take action.

"If we're not going to get an inquiry then we'll get Mr. Mulroney back and put the questions to him in person,'' Martin declared this week.

He's expected to have the backing of Liberal and Bloc Quebecois MPs when the motion comes up for debate at committee Thursday. But Conservative MPs will likely resist the move and could try to filibuster the initiative.

Mulroney has already testified once before the panel, but refused an invitation to return for a second session in the spring. MPs have the power to issue a subpoena forcing his appearance if they wish.

Spokesmen for Mulroney had no comment Wednesday, saying they want to see how Martin's motion plays out.

Mulroney has admitted accepting $225,000 from Schreiber to promote a project to build German-designed light armoured vehicles in Canada for export. He says he tried to line up support among foreign political leaders whose countries might become customers for the vehicles.

Schreiber says $300,000 changed hands _ delivered in cash-stuffed envelopes in hotel rooms. He also claims Mulroney was supposed to lobby the Canadian government rather than foreign leaders, an arrangement that could have put him in violation of federal ethics rules.