DUBLIN—Northern Ireland’s power-sharing government was plunged into crisis Monday as the senior Catholic leader announced he was quitting in a showdown with his Protestant colleague that could unravel a central achievement of peacemaking.

Deputy First Minister Martin McGuinness, the former Irish Republican Army commander who has helped to lead the unity government for nearly a decade, said in his resignation letter that he intends to trigger early elections in protest against his power-sharing partner, First Minister Arlene Foster.

Foster will be forced from office, at least temporarily, if an election is called.

In his resignation letter provided to The Associated Press, McGuinness accuses Foster of ignoring “a public mood that is rightly outraged at the squandering of public money and allegations of misconduct and corruption.”

The government, formed under terms of Northern Ireland’s Good Friday peace accord of 1998, requires support from McGuinness' Irish nationalist Sinn Fein party and Foster's Democratic Unionists, who represent the British Protestant majority.

But tensions between Sinn Fein and the DUP have come close to a breaking point several times. And from the shadows, IRA splinter groups opposed to the outlawed group's 2005 disarmament and renunciation of violence still seek to sow division and disorder.

In recent months, McGuinness repeatedly called on Foster to step aside from her role while lawmakers investigate her alleged mismanagement of a government “green energy” program that Foster eventually suspended as part of her previous Cabinet post.

Lawmakers estimate that the program could cost taxpayers more than 500 million pounds ($600 million U.S.) in inefficient subsidies.

Foster, who became Northern Ireland's first female leader a year ago, has rejected calls from other parties to relinquish the post and accuses her critics of misogyny.

She called McGuinness' resignation "not principled," and warned it would leave Northern Ireland without an administration "at precisely the time we need our government to be active."

But opposition to Foster has been growing. Lawmakers from all other parties voted for Foster's temporary removal in a Northern Ireland Assembly no-confidence vote last month. She survived solely because of backing from Democratic Unionist colleagues.

“That position is not credible or tenable,” McGuinness said in his resignation letter addressed to Assembly Speaker Robin Newton.

McGuinness said Sinn Fein would refuse to nominate a successor to fill his position, which wields equal power to Foster’s in a system designed to promote consensus across the community divide.

If the position goes unfilled, the 108-member Assembly — elected barely eight months ago — would be dissolved for a fresh ballot. The next election normally would happen in May 2021.

Britain's minister responsible for overseeing Northern Ireland, James Brokenshire, said he would be obliged to call an election if Sinn Fein didn't nominate a successor within the next seven days.

During a news conference in his Stormont Castle office, McGuinness denied that his resignation was linked to health problems that have caused him to skip meetings and foreign trips. Sinn Fein has declined to specify McGuinness' ailment.

McGuinness, 66, appeared to have lost weight and looked drawn. He spoke in an unusually weak, at times slurring tone.

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"My health has absolutely nothing to do with this whatsoever," McGuinness said.

McGuinness said power-sharing would not be easily revived.

"If the DUP think in the aftermath of an election they're going to step back into ministerial positions short of resolving the critical issues," he said, "then they're living in a fool's paradise."