A long-time Conservative MP has quit the Tory caucus after being charged under the Canada Elections Act – a fresh ethics embarrassment for Stephen Harper as he struggles to move beyond the Senate expenses scandal.

Dean Del Mastro resigned from the Conservative government caucus on Thursday and was stripped of his duties as a parliamentary secretary.

Mr. Del Mastro, MP for Peterborough, Ont., is facing four charges in connection with allegations that he exceeded his campaign spending limit and filed a false accounting of the expenses incurred to win office in the 2008 federal election.

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The development is unwelcome for Mr. Harper, who was elected promising a new era of ethics and accountability.

Until recently, Mr. Del Mastro was parliamentary secretary to the Prime Minister, and was once a key spokesman for the Conservative government in the House of Commons, defending the Conservative Party on a slew of Elections Canada controversies from robo-calls fraud in Guelph, Ont., to the so-called in-and-out financing scandal.

The charges were filed in the Ontario Court of Justice in Peterborough on Thursday; the allegations have not been proven in court.

They follow a lengthy investigation by Elections Canada.

"In our electoral system, it is fundamentally important that the spending and contribution limits enacted by Parliament be respected. It is also essential that the reports and information provided to Elections Canada be accurate and truthful," said Yves Côté, Commissioner of Canada Elections. "We will continue to be vigilant to ensure that these rules are observed."

Mr. Del Mastro, first elected in 2006, issued a statement rejecting the allegations and saying he looks forward to defending himself in court.

He said he remains dedicated to the "people of the Peterborough riding" and still backs the Harper government's policies.

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"While it is my full intention to continue to support the government's economic agenda and the principles for which it stands, I have advised caucus leadership that it is my intention to step out of caucus until this matter is resolved," he said in a statement.

Back in June, Mr. Del Mastro, frustrated by the length of the investigation, challenged Elections Canada to lay charges or close the case.

Until Thursday, the MP was parliamentary secretary to the Minister of the Canadian Northern Economic Development Agency, for the Federal Economic Development Agency for Southern Ontario, and for the Federal Economic Development Initiative for Northern Ontario. He gives up the $16,000 annual salary that goes with the parliamentary secretary job.

The Conservatives have been trying to demonstrate they have a busy agenda for the return of the Commons in October. Opposition parties plan to shift the spotlight back to the Senate controversy, in which Nigel Wright had to resign as Mr. Harper's chief of staff, and the expense claims of three Harper Senate appointees came under scrutiny.

"For years, whenever there has been scandal with this government, the Prime Minister has attempted to deflect any connection or any knowing all the way to the point where his Chief of Staff is under investigation," NDP House Leader Nathan Cullen said. "I don't know how the Prime Minister with a straight face can turn to Canadians and say he's here to clean up a system that needed fixing."

Mr. Del Mastro is charged with exceeding the limit that a candidate can contribute to his own campaign when he allegedly paid an election expense of $21,000 from his own pocket. The personal limit was $2,100.

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Both the MP and Richard McCarthy, his official agent in the 2008 campaign, are charged with exceeding the $92,655.79 election expenses limit for the Peterborough race. They are charged with submitting an electoral campaign return that omitted to report a contribution, and election expense, of $21,000, and instead reported an expense of only $1,575. This, Elections Canada alleged, was submitting a material statement they knew or should "reasonably … have known" was false or misleading.

In Elections Canada court documents filed in 2012, an investigator for the watchdog alleges Mr. Del Mastro paid the Ottawa-based Holinshed Research Group $21,000 for calls to voters that was not declared in the MP's 2008 campaign finance return.

If found guilty, Mr. Del Mastro and Mr. McCarthy could be hit with hefty penalties. If the Crown proceeds by way of summary conviction, an offender could face fines of as much as $2,000 or up to a year in jail – or both. If the Crown proceeds by indictment, the penalties can reach a maximum of $5,000 or five years in jail – or both.