The Prime Minister's Office attacked the credibility of the ethics commissioner Friday night after he announced an investigation into conflict of interest allegations against Stephen Harper.

Ethics commissioner Bernard Shapiro said he will look into what influence Harper wielded to convince former Liberal cabinet minister David Emerson to cross the House of Commons floor.

"The Prime Minister is loath to co-operate with an individual whose decision-making ability has been questioned, moreover who has been found in contempt of the House," said Sandra Buckler, the prime minister's director of communications.

Prime Minister Stephen Harper's conduct in the Emerson affair will be reviewed. (CP file photo)

"This Liberal appointee's actions have strengthened the Prime Minister's resolve to create a truly non-partisan ethics commissioner, who is accountable to Parliament."

Emerson ran as a Liberal in the campaign leading up to the Jan. 23 general election, but was a surprise appointment to Harper's Conservative cabinet on Feb. 6.

In a letter to three MPs who complained about the switch, Shapiro said he will issue one report on the conduct of both Harper and Emerson, who is now international trade minister.

Bernard Shapiro, the federal Ethics Commissioner. (CP file photo)

• FROM FEB. 11, 2006: 'I won't quit,' embattled Emerson tells CBC

"Although the subject of this inquiry is the prime minister, given that the actions of...Harper and Emerson in this incident were intertwined, questions will no doubt be raised during the course of the preliminary inquiry on the conduct of Mr. Emerson as well," Shapiro said in the letter, a copy of which was obtained by the Canadian Press.

• INDEPTH: Role of the ethics commissioner

But the PMO accused Shapiro of having a double standard when it comes to investigating Conservative MPs. It noted that he refused to investigate Liberal Tony Valeri's landholdings during the election campaign on the grounds the commissioner couldn't act between sittings of Parliament.

Based on that decision, the PMO is arguing that Shapiro has no business investigating something that happened in between the dissolution of the last Parliament and the commencement of the next one.

Shapiro has been criticized in the past. Former NDP MP Ed Broadbent accused him of having "extraordinarily serious credibility problems" after a series of questionable reports last year.

Shapiro was found in contempt of the House by a parliamentary committee regarding his investigation of a Tory MP. The committee concluded he violated MP code of conduct rules and also made inappropriate comments in a media interview, but it recommended no sanctions.

Emerson's switch to the Tories has upset many residents of his Vancouver-Kingsway riding.

Emerson has since formally apologized to his constituents, and has promised to run as a Conservative in the next general election.

The Vancouver-Kingsway riding has only voted Conservative once, in 1958.

In the Jan. 23 election, Emerson won just over 43 per cent of the vote for the Liberals, compared to 33.6 per cent for the second-place NDP candidate, Ian Waddell, and 18 per cent for Conservative Kanman Wong.

Emerson was the head of B.C. forestry giant Canfor when the Liberals recruited him as one of their "star" candidates for the 2004 election. Following his victory in Vancouver Kingsway, the 60-year-old was appointed as former prime minister Paul Martin's industry minister.

Emerson's previous jobs include deputy minister of finance for B.C., president of Western and Pacific Bank of Canada (now Canadian Western Bank) and head of the B.C. Trade Development Corp.