OTTAWA - Prime Minister Stephen Harper has given his special adviser an April 4 deadline to set the mandate for a public inquiry into the Mulroney-Schreiber affair.

David Johnston, president of the University of Waterloo, has been asked to use his preliminary report on the matter, delivered Jan. 9, as well as witness testimony from the Commons ethics committee to set parameters for the new probe.

Harper has long said an inquiry wouldn't begin until the ethics committee had finished its work on the relationship between former Tory prime minister Brian Mulroney and German-Canadian arms lobbyist Karlheinz Schreiber.

The committee finished its hearings earlier this month and is working on a final report that's expected to state simply that it couldn't resolve all the issues raised and will reiterate the need for a more thorough investigation.

Mulroney has admitted taking $225,000 from Schreiber after he left office in 1993 to lobby international leaders about a proposal to build German-designed light armoured vehicles in Canada for export.

Schreiber claims the total was $300,000 and Mulroney was supposed to lobby the Canadian government -- something that could have put him in violation of federal ethics rules.

Johnston's preliminary report in January suggested a relatively narrow inquiry, but the contradictory testimony at the ethics committee since then has led opposition MPs to demand a wider-ranging probe.

They want the investigation to include not just the armoured vehicle lobbying, but also an earlier deal that saw Air Canada buy European-made Airbus jets while Mulroney was still in power.

They also want the probe to include a look at whether Mulroney should repay a $2.1 million libel settlement reached with the former Liberal government in 1996. Mulroney didn't disclose the full extent of his dealings with Schreiber before reaching the settlement.

John Gomery, the retired judge who won fame for his investigation of the federal sponsorship scandal, waded into the debate last week. He argued that any "politically charged'' inquiry, like the one into the Mulroney-Schreiber affair, should give the presiding judge as wide a mandate as possible to follow up on unexpected leads that may turn up.