Prime Minister Stephen Harper forced Sen. Pamela Wallin out of the Conservative caucus after learning the preliminary findings of an audit looking into her travel expenses, a source has told CTV News.

Wallin issued a statement Friday evening saying she has recused herself from the caucus as she awaits the results of the external audit.

But a source told CTV’s Ottawa Bureau Chief Robert Fife that the audit has already raised serious questions about Wallin’s spending, which involves hundreds of thousands of dollars.

Insiders told Fife that Wallin repaid $25,000 before the forensic audit began. She has since returned about $15,000 more to taxpayers, but sources say she will likely have to give more money back.

As the controversy over senate expenses grows, Fife reported there’s word Prime Minister Stephen Harper may prorogue Parliament in early June.

Wallin said Friday she has been cooperating “fully and willingly” with the auditors since December.

“I had anticipated that the audit process would be complete by now, but given that it continues, I have decided to recuse myself from the Conservative Caucus and I will have no further comment until the audit process is complete,” she said in her statement.

When CTV News first disclosed Wallin’s suspicious travel spending in February, Harper defended the senator in the House of Commons.

“I have looked at the numbers. Her travel costs are comparable to any parliamentarian travelling from that particular area of the country,” he said, referring to Wallin’s home province of Saskatchewan.

Alarmed by what they’ve discovered so far, the auditors are now scrutinizing all of Wallin’s travel expenses since 2009 – the year she was appointed to the Senate.

Auditors had at first only been probing about a year’s worth of travel expenses.

Once the audit is complete, it will be sent directly to the RCMP, a well-informed insider told Fife.

Sen. Marjory LeBreton, government leader in the Senate, issued a terse, one-line statement Friday to acknowledge Wallin’s decision to leave the Tory caucus.

"Senator Wallin has informed me that she has resigned from Caucus to sit as an independent,” LeBreton said.

Wallin’s news comes a day after Sen. Mike Duffy also stepped down from the Conservative caucus over an ongoing expense claim scandal.

Some $321,000 in travel expenses claimed by Wallin are currently the subject of an external audit by the same firm that probed housing expense claims by Duffy, Sen. Mac Harb and Sen. Patrick Brazeau.

When the audit results were released on May 9, LeBreton said that auditing firm Deloitte had requested more time to review Wallin’s travel expenses. The internal economy committee granted that request, she said.