Work and Pensions Secretary Peter Hain could face a police inquiry into the late declaration of donations to his failed Labour deputy leader campaign.
Mr Hain is facing two separate investigations into donations
It is one of the options being looked at by the Electoral Commission, which is already investigating the matter.
Mr Hain, who is also Welsh secretary, failed to declare on time £103,000 in donations to last year's campaign, but put the blame on poor administration.
He says he has been "open and clear" and will co-operate with any inquiry.
Neath MP Mr Hain, who came fifth in the six-candidate 2007 Labour deputy contest, is also facing a separate inquiry by Parliamentary Standards Commissioner John Lyon.
BBC political editor Nick Robinson said Electoral Commission staff were feeling "real frustration" they had a choice only between giving Mr Hain a "slap on the wrist" for breaking electoral rules, or calling in the police - which could lead to a possible court hearing and a potential fine of up to £5,000.
Under the law, political donations need to be reported within 60 days of being offered.
Meanwhile, the BBC has learned that Tory leader David Cameron failed to tell the Electoral Commission about three free flights he took during his leadership campaign in 2005.
The donated flights are registered in David Cameron's name in the Commons register of members' interest, but do not appear in the Electoral Commission's record.
The commission says a donation of more than £1,000 must be declared and it is estimated these flights would have cost between £3,000 to £8,000.
However Mr Cameron's office said the costs could be split between the number of people on board, keeping the amount for Mr Cameron below £1,000 on at least one of the flights.
Electoral commissioners will meet later this week, when they could discuss the donations to Mr Hain's campaign.
They could consider a report from officials at the commission and then decide whether or not to request a police investigation.
The commission is expected to produce its initial findings by the end of the week, with a full report coming later.
Meanwhile, officials at the standards commissioner's office say he believes there are grounds for an investigation, following a complaint by Monmouth Tory MP David Davies.
Mr Hain could face suspension from the Commons if he is found to have flouted the rules on members' interests.
Prime Minister Gordon Brown has given his backing to Mr Hain, saying he was doing a "great job", but he said his fate would be decided by the two inquiries.
Mr Hain said it was "absurd" to suggest he had attempted to hide anything.
He told the BBC: "I've been open and clear about this from the beginning when I discovered these donations had been made late.
"I told the public, I told you in the media, I told the relevant authorities - the parliamentary commissioner for standards and also the Electoral Commission."
Mr Hain also faces questions over the role of a think-tank, the Progressive Policies Forum (PPF), in channelling donations.
The PPF employs no staff and has not published any research since it was set up in December 2006, three months after Mr Hain launched his campaign.
The Tories have also been criticised after it was reported that shadow chancellor George Osborne did not declare £487,000 of donations to the Commons register of members' interests.
Parliamentary Standards Commissioner Mr Lyon has received a complaint about that omission, his office said.
Conservative leader David Cameron said the two cases were "completely different" because Mr Osborne's donations had been registered with the Electoral Commission.
Of Mr Hain, he said: "He has a choice: he can get out there and explain himself, or I think he will have to leave the cabinet."