London's Metropolitan Police will today begin its investigation into secret donations to the Labour Party.
Scotland Yard has been asked to take over the inquiry into revelations that property developer David Abrahams had channelled £600,000 to the Labour Party through intermediaries.
Electoral laws require those making donations for others to give details of the source of the money. Mr Brown said the money from Mr Abrahams had not been lawfully declared and would be returned.
Mr Abrahams said he didn't realise that using middlemen to donate to Labour was against the law and that he had merely wanted to avoid publicity. However, Labour's general secretary Peter Watt resigned on Monday after admitting he knew Mr Abrahams had donated through associates.
On Wednesday, the Electoral Commission, parliament's watchdog for political funding, said it had referred the matter to police after discussions with detectives and prosecutors.
A number of senior Labour figures have become embroiled in the row since it erupted. Labour deputy leader Harriet Harman has said she acted in "good faith" in accepting a £5,000 donation from Mr Abrahams through a go-between.