Tory leader David Cameron says Labour has been either "utterly dysfunctional" over donations to the party or the whole truth has not come out.
He questioned Labour's chief fundraiser Jon Mendelsohn's claim he only knew of David Abrahams use of proxy donors in September but did not tell MPs.
Lib Dem Chris Huhne has spoken to Durham police to raise concerns about a planning deal involving Mr Abrahams.
Labour's Scotland leader is also under pressure over an unlawful donation.
During his last monthly media conference of the year, Mr Cameron queried the behaviour of Mr Mendelsohn in the row over Labour's donations.
"We are being asked to believe that Mendelsohn was hired by Gordon Brown, that he found out about these secret donations," he told reporters.
"He somehow thought it was legal, but at the same time was deeply unhappy about it but told nobody.
"Either this organisation is utterly dysfunctional or we are not being given the whole truth."
Property developer Mr Abrahams says Mr Mendelsohn knew in April how he planned to make his donations.
Mr Mendelsohn said the allegations were "fictional" and that he only learned of the practice in September.
Gordon Brown has vowed to co-operate with a police probe into the affair.
He is seeking to get the focus back to the business of government with plans being unveiled for reform of cancer services and treatment.
He said: "We know that donations were given wrongly, in breach of the electoral law. An inquiry is taking place, the police are looking at this...We will get this sorted out - we'll reform the electoral law."
Mr Cameron also stressed that he would be happy to rejoin cross-party talks on party funding, as long as Labour tackled the issue of trade union donations.
He said there should be a £50,000 cap on donations to parties from trade unions, individuals or businesses, and general election spending should be lowered from £20m to around £15m.
Lib Dem leadership contender Mr Huhne has spoken to Detective Chief Superintendent Ian Scott, head of Durham CID, about his concerns over a land-deal involving Mr Abrahams.
It is alleged that Mr Abrahams had a ruling against plans for a business park, in which he had an interest, overturned in the same year as he made a large donation to Labour.
Both he and the Labour Party deny wrongdoing but Mr Huhne wants an investigation.
"We had a good and thorough conversation in which I spelled out my concerns that British politics should not just be clean but seen to be clean," he told the BBC.
"It's absolutely crucial, particularly when we have evidence of hidden donations which were unlawful, that benefits which subsequently materialised for the real donor should be investigated to ensure that they were totally proper and above board and that there was no untoward influence."
Labour's chief whip Geoff Hoon said Mr Brown had been "completely shocked" and "really upset" by the revelations.
Health Secretary Alan Johnson admitted it had been "a dreadful week for the party".
"There's nobody in the Labour Party, Gordon Brown through to the person who joined last week, who is not concerned and deeply upset and even depressed about what has happened over the last week - it's been a dreadful time," he told BBC Breakfast.
By law, anyone donating more than £5,000 must be identified and relevant details about them disclosed.
The donation row was prompted by the revelation that property developer David Abrahams had given Labour more than £650,000 since 2003 under four other people's names.
He said he used the four - two who worked for him, his solicitor and the wife of one of his staff - to give his donations because he wanted to protect his privacy.
Labour's general secretary Peter Watt resigned after admitting he knew about the arrangement but thought it was legal.
Meanwhile, Labour's leader in Scotland, Wendy Alexander, rejected any suggestion of "intentional wrongdoing" over a £950 donation from Jersey-based businessman Paul Green during her leadership campaign.
It has emerged that the donation broke rules outlawing donations from people based outside the UK.