Peter Hain, former work and pensions secretary, has been interviewed by police over his failure to register political donations.
The interview occurred weeks ago, with officers investigating details of his campaign to be deputy Labour leader.
A file could be sent to the crown prosecution service within the next few weeks, specifying if a prosecution is possible over allegations he failed to declare £103,000 to the Electoral Commission.
The money was declared four months after the end of his campaign, way past the 30-day requirement for donations over £1,000.
The £103,000 figure constitutes 17 different donations. Mr Hain went to the Electoral Commission in January with details of the donations, five of which came through a seemingly functionless thinktank called the Progressive policy Forum.
The news comes two days after Wendy Alexander, Labour's leader in Scotland, resigned over less serious allegations which Hollyrood's standards committee thought significant enough to impose a one-day ban from the Scottish parliament.