The allegation centres on an incident that allegedly took place in Westminster in 2016, at the heart of the political establishment. According to the BBC, the woman has been interviewed by PSNI officers, who then shared her claims with the Metropolitan Police, as the alleged incident occurred in London.
In a statement, the Metropolitan Police said: "On Tuesday 13 October, the Met received a report referred to us by the Police Service of Northern Ireland relating to an allegation of rape in 2016, by a man now aged 63. The allegation relates to one victim who is being supported by specialist officers. The investigation is in its early stages. No arrests have been made and enquiries are ongoing."
The story was first reported by the Belfast Telegraph in July, which said a woman from Northern Ireland alleged she had been sexually assaulted by Donaldson in the DUP offices at Parliament in 2016. She claimed he attempted to get her to drink vodka shots in his office before trying to kiss her and put his hand up her skirt. On Saturday, the newspaper reported that the Met was set to investigate and interview Donaldson about those accusations.
In response to further questions from the BBC, it emerged the same woman also alleges Donaldson raped her. Earlier this year, a BBC Northern Ireland Spotlight documentary raised concerns about the former MP's behaviour in London.
Donaldson was sentenced last week for a series of child sex offences against two victims, including one count of rape, that took place between 1985 and 2008. He is appealing his convictions.