BELFAST (Reuters) -The leader of Northern Ireland's largest unionist party Jeffrey Donaldson stepped down on Friday after confirming he had been charged with "allegations of an historical nature", the Democratic Unionist Party (DUP) said. Donaldson 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. "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.