The Metropolitan Police did not identify the 63-year-old politician but said in a statement it is investigating a report passed on by officers in Northern Ireland.
It said: “On Tuesday October 13, the Met received a report, referred to us by 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 inquiries remain ongoing.”
Earlier this month, Donaldson was given a 15-year sentence for a series of historical sex offences against two women when they were children.
He was convicted of one count of rape, 13 indecent assaults, and four counts of gross indecency, committed between 1985 and 2008.
Donaldson’s solicitor has previously confirmed that papers have been lodged for an appeal.
