CBC News has learned that the "serious allegations" Prime Minister Stephen Harper referred to last year in connection with former Conservative MP Helena Guergis included unsubstantiated claims of fraud, extortion and involvement with prostitutes.

But a letter written by a Harper aide and obtained by CBC News states that the Prime Minister's Office learned the allegations had been made by a Toronto private investigator. The letter also reveals the allegations were not based on any hard evidence.

Harper has never explained what the allegations were against Guergis, who was later cleared by the RCMP of any wrongdoing. The Ontario MP, who was ousted from cabinet, caucus and the party in April 2010, also has claimed she was never personally told what the allegations were, something the Tories deny.

LIVE COVERAGE: CBC News will have live coverage of Helena Guergis's press conference at 11 a.m. Friday online and on CBC News Network.

But CBC News has obtained the letter sent by Harper's principal secretary, Ray Novak, to RCMP Commissioner William Elliott regarding Guergis and her husband, former Tory MP Rahim Jaffer.

"The allegations are numerous and include fraud, extortion, obtaining benefits by false pretences and involvement in prostitution," Novak wrote in a letter dated April 9, 2010.

But Novak wrote that the Prime Minister's Office has "no first-hand knowledge of these allegations."

Instead, Novak said that the PMO had "become aware of the specifics of the allegations" made by Toronto-based private investigator Derrick Snowdy.

Novak wrote that the PMO had not communicated directly with Snowdy and that communication was conducted through the Conservative Party's legal counsel, Arthur Hamilton.

A week later, Hamilton was interviewed by two RCMP investigators about the allegations. CBC News has obtained a redacted transcript of the interview.

Hamilton raised allegations of "Jaffer and Guergis on tape, partying with hookers and doing cocaine" and that "not only does Helena tolerate Jaffer hanging out with, with escorts, and prostitutes, but there's apparently video of her snorting cocaine off the breasts of a prostitute."

"I've made no verification of that," Hamilton added during the interview, citing newspaper articles as an additional potential source of information.

At that point in the interview, the RCMP asked Hamilton if he was aware of anybody who could back up his claims.

"I am not," he said.

But Hamilton did confirm to the RCMP investigators that he had spoken to Snowdy.

When contacted by CBC News, Hamilton refused to comment.

In an email to CBC News late Thursday, Dimitri Soudas, Harper's director of communications, wrote: "Information was given to us. We passed that information the authorities. The authorities dealt with it."

A month after Hamilton spoke to the RCMP, Snowdy, appearing before a parliamentary committee, said under oath that he had "no evidence or information with respect to the conduct of Ms. Guergis in my possession or knowledge."

After a three-month investigation, the RCMP, in a reply to Novak, said in July 2010 they had concluded there was no "evidence to support a charge under the Criminal Code."

Guergis is currently running as an Independent MP candidate.

Jaffer had been accused of trying to use his connections to former Conservative colleagues in the government to gain access to a federal green infrastructure fund for several businesses.

Jaffer, who was not registered as a lobbyist, denied conducting unregistered lobbying or receiving a penny of federal funds.

Guergis is holding a news conference about the issue at 11 a.m. Friday.