Prime Minister Gordon Brown is set to face a grilling over Labour's £600,000 "secret" donations at his monthly press conference.He will attempt to restore public faith just hours after the party's most senior official, General secretary Peter Watt, quit after admitting he knew a wealthy donor had hidden his identity from regulators.His departure marks yet another low point for Mr Brown who had already suffered his worst week since taking power from Tony Blair in June.The loss of millions of child benefit records, the ongoing Northern Rock crisis and attacks from former army chiefs have left his administration reeling.And the latest opinion poll has put the Tories 13-points in front - its biggest lead for nearly 20 years and enough to secure a 64-seat Commons majority.The Prime Minister has insisted problems at HM Revenue & Customs (HMRC) and the ailing bank Northern Rock were caused by factors outside the Government's control and he has urged voters to judge him on his long-term record of delivering economic stability.But the latest problem strikes right at the heart of the Labour machine and is bound to revive the sort of "sleaze" concerns that engulfed Tony Blair's final months in office.Mr Watt's resignation came as it emerged the sum involved was more than £600,000 - half as much again as the £400,000 that had been originally reported.He quit after admitting he knew wealthy donor David Abrahams channelled a series of large injections to party coffers via friends and colleagues to keep his own name secret.He said he had believed he had made all the necessary legal declarations but had now been informed by lawyers of "additional reporting requirements".The Electoral Commission has launched a formal inquiry into whether any laws have been broken - with one former sleaze watchdog warning it could become a police matter.But Mr Abrahams said Mr Watt should not have left his job and insisted he had not done "anything wrong or untoward" in making the donations via third parties.Electoral Commission records show a series of large donations from Mr Abrahams' associates Ray Ruddick and Janet Kidd, culminating in gifts of £80,000 from each on a single day in July.Last night a third individual, Newcastle-based solicitor John McCarthy, was revealed to have also made a series of donations on his behalf totalling more than £200,000 since 2004.They are collectively the third biggest donors during Mr Brown's tenure as Prime Minister.© Independent Television News Limited 2007. All rights reserved.