A Canadian senator at the center of an expenses scandal sought medical leave on Thursday as he and two colleagues were set to be sacked from government.

Senator Mike Duffy said in a statement that he would be absent from work due to health concerns.

The Senate has been asked to consider a motion calling for the suspension without pay of Duffy, as well as fellow Senators Pamela Wallin and Patrick Brazeau.

The trio -- all appointed to the upper house by Conservative Prime Minister Stephen Harper -- face a criminal probe after an audit revealed "troubling" expense claims.

All three have been stripped of their political resources but that was not enough to quell an uproar which has been a drag on the Conservatives' popularity -- now at an all-time low, according to polls.

Duffy is also being investigated for having accepted a Can$90,000 check from Harper's chief of staff to help the lawmaker repay funds he had wrongly claimed as Senate expenses.

Harper's right-hand man, Nigel Wright, resigned in May after revealing the payment. The opposition on Thursday accused Harper of having misled the House of Commons when he said he knew nothing about the transaction.

In a statement, Duffy said he had been hospitalized for three days in August for "unstable angina" and an examination this week indicated the disease may have progressed.

Duffy had heart surgery in 2006. "My GP (general practitioner) suggests that to avoid further stress on my system, I stay off work until I get the all-clear from my medical team," he said.

Brazeau also faces criminal charges for alleged domestic violence.

Senators will vote on the motion on Tuesday, an official said.