BELFAST (Reuters) - Irish nationalists Sinn Fein became Northern Ireland's largest party in the British parliament for the first time on Friday, capitalising on a poor election for its main unionist rival to cross off another milestone in its campaign to end British rule.

With 17 of the 18 seats declared, the former political wing of the Irish Republican Army (IRA) had retained the seven seats won at the last election. The Democratic Unionist Party (DUP) were second on four seats and cannot catch Sinn Fein.

That was down on the eight seats the DUP won in 2019 and the losses included Ian Paisley Jr., the son of former Northern Ireland First Minister and party founder Ian Paisley. The father and son had held a seat at Westminster since 1970.

The DUP, the largest pro-British party in Northern Ireland, fought the election just three months after the shock resignation of then leader Jeffrey Donaldson after he was charged over historical sex offences.

Sinn Fein's victory marked an electoral clean sweep for the party, which in 2022 became the first nationalist party to win the most seats at the regional assembly since Northern Ireland's creation in 1921, and won at local council polls a year later.

It also took over as Northern Ireland's largest party in Westminster despite its long-standing policy of not taking up its seats there.

(Writing by Padraic Halpin; Editing by Conor Humphries)

By Amanda Ferguson