Some of Prime Minister Stephen Harper's closest former advisers will appear as Crown witnesses in the trial of suspended senator Mike Duffy, CBC News has learned.

And those subpoenaed to testify include several Conservative MPs, according to multiple sources familiar with the Crown's witness list.

Duffy faces 31 charges of bribery, fraud and breach of trust at a trial scheduled to begin in five weeks. That trial will almost certainly dominate the federal political agenda in the first half of an election year.

One of the key witnesses at the trial who has been subpoenaed by the Crown prosecutor is the prime minister's former chief of staff Nigel Wright. He now works in London, but has previously been interviewed by the RCMP and handed over hundreds of emails.

The Crown will look to the witnesses to help make its case in relation to $90,000 Duffy accepted from Wright to repay ineligible living and travel expenses and allegations Duffy double-dipped while claiming expenses with the Senate for partisan events.​

Court documents show Wright was consumed with managing the potential political damage of the Duffy expense scandal for weeks in early 2013, and, after trying to convince Duffy to reimburse the Senate for improperly claimed expenses, finally decided to give Duffy $90,000 to repay them.

Wright has maintained that Harper did not know the details of the agreement. In the emails from the RCMP, Wright says, "The PM knows, in broad terms only, that I personally assisted Duffy when I was getting him to agree to repay the expenses."

Emails expected to be key

Other former Prime Minister's Office insiders who have been subpoenaed include David van Hemmen, Wright's former executive assistant, and Benjamin Perrin, who acted as the prime minister's lawyer. Both men no longer work for the PMO.

Emails obtained by the RCMP say Perrin acted as the go-between for the PMO and Duffy's lawyer to try to end the crisis.

Perrin's emails have not been released publicly, but there are questions about one of Wright's emails to him and others during negotiations around Duffy's repayment of the expenses, which at the time were believed to be about $32,000.

According to documents filed in court by the RCMP, Wright at one point suggests he wants to speak to Harper before "everything is considered final," and then later emails, "We are good to go from the PM."

The RCMP's lead investigator, Cpl. Greg Horton, says in the court documents, "I am not aware of any evidence that the prime minister was involved in the repayment or reimbursement of money to Senator Duffy or his lawyer."

MPs called to testify

CBC News has also learned a number of Conservative MPs will be called to testify by the Crown.

Conservative MP Barry Devolin is one of several Conservative MPs who have been subpoenaed to testify at the Duffy trial. (Adrian Wyld/Canadian Press)

Those MPs will be asked about fundraising by the senator when he was still a hot commodity for the Conservative Party. One of the MPs is Barry Devolin, who is expected to be asked to testify about an event he held in his Haliburton-Kawartha Lakes riding with Duffy.

Duffy regularly travelled across the country to help raise money for the party. The RCMP alleges he filed Senate expense claims while conducting personal or partisan trips.

Sources say MPs who have been subpoenaed have been asked to co-operate fully.

Duffy has maintained his innocence and said repeatedly he is looking forward to a fair trial.

Senators who were part of the audit and report process will be called to testify, including Marjory LeBreton, David Tkachuk and Carolyn Stewart-Olsen.

In a statement Monday, a spokesperson for the PMO said," The Prime Minister's Office will continue offering every possible assistance to the RCMP's investigation against Mr. Duffy."

Duffy, 68, was appointed in 2008 as a Conservative senator, representing Prince Edward Island, and began sitting in the Senate in January 2009. He resigned from the Tory caucus on May 16, 2013, and sat as an Independent until his colleagues voted to suspend him without pay for two years on Nov. 5, 2013.

The trial is set to begin April 7 and is scheduled to last 41 days.