LONDON - Opposition parties stepped up attacks on Prime Minister Gordon Brown's Government on Sunday, demanding to know who knew about illegal secret donations to the Labour Party.

The property developer at the centre of the scandal reportedly alleged that 10 Labour Party officials were aware of his arrangement to give more than 600,000 pounds to the party through go-betweens.

Law requires people donating on behalf of others to disclose the source of the money.

An ICM poll for the News of the World newspaper showed the donations scandal and a series of other mishaps were seriously undermining the popularity of Brown's five-month-old Government with the Conservatives enjoying an 11-point lead.

Labour's general secretary has resigned over the scandal and the police have been called in to investigate -- just months after detectives ended a party funding probe that clouded the final months in office of Brown's predecessor, Tony Blair.

Brown took over from Blair pledging to restore trust in government. Recently, however, the Government has admitted to losing half the population's personal data and has had to deal with the country's first bank run in more than a century.

"I think what we've got to find out is who knew about this in the Labour Party ... I find it pretty incredible really that only one or two people knew about it," Conservative leader David Cameron told the BBC.

"Are we really meant to believe that Mr Mendelsohn didn't know what the law was, didn't tell anyone about it, (that) Brown knew nothing?" Cameron said, referring to Brown's chief fundraiser Jon Mendelsohn. "It beggars belief."

The Liberal Democrats' acting leader, Vince Cable, said in a statement: "This sleazy affair is getting worse by the day. Who are the people that knew about these donations, and why didn't they do anything?"

Property developer David Abrahams says he made the donations anonymously because he did not want to draw attention to his wealth. He said he acted in good faith.

Brown has acknowledged Labour accepted illegal donations and has ordered them to be returned. He has said he knew nothing about them until a week ago.

According to The Sunday Times, Abrahams alleged that 10 Labour Party officials knew about how he made donations. A Labour Party spokesman could not immediately be reached for comment.

Abrahams says he told Mendelsohn about his method of making donations in April. Mendelsohn denies it, saying he only learned about in September when he was told the arrangement was legal.

Labour ministers called for a new drive to reform political party funding. "Let's see if we can't get around a table, get some agreement (and) get all the political parties on the same playing field," Communities Secretary Hazel Blears told the BBC.

Cameron said he would support a cap of 50,000 pounds on individual donations, but it would have to apply to trade unions as well.