DUBLIN — Northern Ireland’s power-sharing government was plunged into crisis Monday as its senior Catholic leader quit in a showdown with his Protestant colleague that could unravel a central achievement of the region’s peace accord.

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

In a copy of his resignation letter provided to The Associated Press, McGuinness accused 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 1998 Good Friday peace accord, requires support from the Irish nationalist Sinn Fein party and Foster’s Democratic Unionists, who represent the British Protestant majority.

Their unlikely partnership has been credited with governing the long-disputed corner of the United Kingdom in relative harmony following the four decades of bloodshed that claimed 3,700 lives.

But tensions between Sinn Fein and the DUP have come close to breaking point several times before. And IRA splinter groups opposed to the outlawed group’s 2005 decision to renounce violence and disarm still plot gun and bomb attacks in hopes of reigniting division and disorder.

In recent months, McGuinness repeatedly had called on Foster to step aside while lawmakers investigate her alleged mismanagement of a government “green energy” program. Lawmakers estimate the program could cost taxpayers more than 500 million pounds ($600 million) in inefficiently distributed subsidy payments.

Foster, who became Northern Ireland’s first female leader a year ago, has rejected calls from other parties to relinquish the post and accused her critics of misogyny. But both male and female lawmakers voted for her to step down in a Northern Ireland Assembly no-confidence vote last month. She survived solely because of backing from her own party.

“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 of deputy first minister, which wields equal power to Foster’s own 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’s ailment.

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

“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.”