The trial of former Democratic Unionist party (DUP) leader Sir Jeffrey Donaldson for alleged sex offences is set to begin in a case that triggered a political earthquake in Northern Ireland.

Donaldson, 63, is charged with rape, gross indecency and other sexual offences spanning 21 years. His wife, Eleanor Donaldson, 60, is charged with aiding and abetting rape and indecent assault and will be subject to a trial of facts.

A heavy security and media presence is expected at Newry crown court for the opening of a trial on Tuesday that is expected to last at least four weeks.

Donaldson, a former MP for Lagan Valley in County Down, was a dominant figure in unionism and a key player at Westminster who helped to broker the post-Brexit Windsor framework on Northern Ireland’s trading arrangements.

Police arrested him and his wife at their home and questioned them in March 2024. He resigned as an MP and DUP leader and was suspended from the party. The case threw the DUP and the Stormont executive into disarray.

He is charged with 18 offences involving two alleged victims. The rape charge alleges he had unlawful sexual intercourse with the alleged victim without her consent on a date unknown between 1985 and 1991. Nine allegations of indecent assault span 1985 to 2006. He is also charged with committing an act of gross indecency towards a child between 2005 and 2006.

Lady Donaldson faces five charges relating to aiding and abetting her husband’s alleged offending. Both deny the charges and have pleaded not guilty.

After several delays to the trial, the judge, Paul Ramsey, ruled last week that Eleanor Donaldson was unfit to stand trial on mental health grounds and would instead face a trial of facts. “The interests of justice require that this case be progressed in a manner that is fair to all parties, but also as expeditiously as such fairness permits,” he said.

A trial of facts tests the evidence but cannot result in a criminal conviction. Lawyers will represent Eleanor Donaldson but she will play no part in the proceedings. The two sets of proceedings will unfold simultaneously, with different legal teams representing the husband and wife.

Northern Ireland’s attorney general, Dame Brenda King, has warned against making social media posts that could infringe victims’ right to anonymity or impact jury members.

“I would remind members of the public that they can be found in contempt of court if something they publish creates a substantial risk that the course of justice in the criminal proceedings would be seriously impeded or prejudiced. A sentence of up to two years imprisonment can be imposed.”

The DUP replaced Donaldson with Gavin Robinson, an MP for east Belfast, who continued the party’s work on the Windsor framework, which Donaldson had negotiated with Rishi Sunak’s Conservative government weeks before his arrest.

The DUP lost the byelection for Donaldson’s vacated Westminster seat to Sorcha Eastwood of Alliance, and will face a fresh test in Stormont assembly elections next year.

Donaldson was born into a Presbyterian family in the fishing village of Kilkeel, became a full-time political activist at 18 and married Eleanor in 1987. He served apprenticeships with Enoch Powell and James Molyneaux of the Ulster Unionist party before defecting to the DUP in 2003. He was knighted for political services in 2016 and became DUP leader in 2021.

The trial’s opening day is expected to focus on jury selection.