Donaldson, 61, is one of the British region's best-known politicians and is Northern Ireland's longest-serving lawmaker in the British parliament, to which he was first elected in 1997. He is also a former member of the Northern Irish Assembly.

"The Party Chairman has received a letter from Sir Jeffrey Donaldson MP confirming that he has been charged with 'allegations of an historical nature' and indicating that he is stepping down as leader of the Democratic Unionist Party with immediate effect," the DUP said in a statement on Friday.

"In accordance with the party rules, the party officers have suspended Mr Donaldson from membership, pending the outcome of a judicial process."

The Police Service of Northern Ireland (PSNI) said earlier on Friday that detectives had arrested and charged a 61-year-old man for "non-recent sexual offences".

A 57-year-old woman was also arrested and charged at the time "for aiding and abetting additional offences". Both are due to appear in court on April 24, police said.

In response to questions from Reuters, the DUP and the police gave no details about the identities of either individual, or the victim. They also declined to disclose what the charges were and what triggered the arrests.

A source close to the case told Reuters that the male suspect was Donaldson, as other media organisations also reported. Reuters was unable to directly verify that.

Donaldson did not respond to Reuters requests for comment.

The DUP appointed deputy leader Gavin Robinson, a fellow member of the British parliament, as interim party leader.

As a member of the London parliament, Donaldson does not hold a position in the Northern Irish power-sharing government, a key part of the region's 1998 peace settlement that only resumed last month following a two-year suspension.

Northern Ireland First Minister Michelle O'Neill, whose Irish nationalist Sinn Fein party is obliged to share power with the DUP under the peace deal, said the power-sharing government was focused on delivering its policy objectives.

"My priority is to continue to provide the leadership the public expect and deserve, and to ensure the four-party executive coalition delivers for the whole of our community now and in the future," O'Neill said in a statement.

Donaldson's resignation comes amid a tumultuous period for the DUP and with elections to the British parliament expected later this year.

Sinn Fein overtook the DUP for the first time at elections to the Northern Irish assembly in 2022, paving the way for O'Neill to become the region's first Irish nationalist First Minister.

Donaldson was the third person to lead the DUP in a matter of weeks when he took over in 2021 after the party in effect dumped both his predecessors.

He also had to face down some opposition from within the party last month when the DUP decided to end its boycott of the power-sharing government over post-Brexit trade rules.

Donaldson brokered a deal with the British government that eased some of the trade barriers with the rest of the United Kingdom that the DUP and many unionist voters said undermined Northern Ireland's place in the union.

All the lawmaker's social media accounts appeared to have been deleted, with Donaldson's handle on the social network X showing the message "This account doesn't exist".

Australian Associated Press