The federal revenue minister has been called to appear before a parliamentary committee to explain a controversial deal between the Canada Revenue Agency and clients of the financial firm KPMG.

The House of Commons Finance Committee passed a motion Thursday calling on Revenue Minister Diane Lebouthillier to appear before the committee along with CRA officials sometime before May 20.

Opposition MPs on the committee want to know why the CRA offered an amnesty to KPMG clients who took part in an offshore tax scheme in the Isle of Man, which the agency itself labelled a "sham."

Details of the deal came to light through a series of investigative stories by CBC News.

The deal allows "high-net worth" clients of the firm to be free from any future civil or criminal prosecution as well as any penalties or fines if they agree to pay their back taxes and a modest interest charge on their offshore investments.

NDP Finance critic Guy Caron says the minister needs to explain what's going on.

He also wants Lebouthillier to clarify comments she made this week in the House of Commons in which she denied any amnesty had been offered.

"That flies in the face of the facts," Caron said.

"This issue of what happened is a question of transparency, it is a question of governance, of openness but also ministerial responsibility."

CRA to scrutinize Isle of Man

The chair of the finance committee, Liberal MP Wayne Easter, could not say exactly when the minister would appear.

Francois-Philippe Champagne, the parliamentary secretary to the minister of finance, said Lebouthillier will appear as soon as her schedule allows.

"The minister has expressed in the House, in question period, actually, her desire to come.

"She's going to come as soon as possible," he said.

Champagne and Lebouthillier were both part of an announcement this week in which the federal government said it would invest $444 million in helping the Canada Revenue Agency crack down on offshore tax havens.

The government's first target for increased scrutiny will be the Isle of Man, where the KPMG financial scheme was based. Champagne vowed that new enhanced enforcement would mean there would be "no safe place in the world to engage in tax evasion and tax avoidance schemes."

The opposition is still waiting to see proof.

"It's hard to know," said the NDP's Caron.

"Conservatives did that in the past. They said we invested this much money to go against tax cheats. And we haven't seen any significant results"