LONDON - Gordon Brown said illegal donations to his party were "completely unjustifiable" on Wednesday as he sought to limit the political fallout from the growing funding scandal.

Brown brushed aside calls at Prime Minister's Questions to call in police over the 600,000 pounds that property developer David Abrahams donated through intermediaries.

"It has got to be investigated as a matter of urgency," Brown told parliament. "Two internal inquiries have been set up within the Labour Party, and the Electoral Commission will investigate.

"I am determined to make sure that political party finances are above board."

Conservative leader David Cameron said the latest setback for Brown's government raised questions about the prime minister's future.

"We have had 155 days of this government, we've had disaster after disaster," he said. "His excuses go from incompetence to complacency and there are questions about his integrity.

"Aren't people rightly asking now: 'Is this man simply not cut out for the job?'"

Electoral laws require those making donations on behalf of others to give details of the source of the money.

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.

"Until Friday, I didn't know it was illegal," Abrahams told the BBC. "If I had known at the time, I would most certainly not have donated in that way."

One of those named in the media as an intermediary, secretary and school lollipop lady Janet Dunn, said she was a lifelong Tory and knew nothing about the payments.

Abrahams also said he received a letter from Labour's chief fundraiser Jon Mendelsohn on Tuesday, thanking him for being "one of Labour's strongest supporters".

Labour's General Secretary Peter Watt resigned on Monday after the row erupted at the weekend. Deputy Labour leader Harriet Harman said she will return 5,000 pounds she received for her leadership campaign.

Labour said it had launched an inquiry on Wednesday.

The fiasco is the latest challenge for Brown, who took over from Tony Blair in June and whose popularity has plummeted since his perceived dithering and lack of nerve in failing to call an early general election in the Autumn.

Since then, the Northern Rock collapse and the loss of two discs with the personal details of half the nation have eroded public confidence in his leadership, polls suggest.

Acting Liberal Democratic leader Vince Cable put the transformation bluntly during Wednesday's parliamentary questions. Brown, he said, had gone "from Stalin to Mr Bean" in a matter of weeks.