Downing Street's top civil servant made an extraordinary plea to staff yesterday to come forward if they had been bullied.
Jeremy Heywood urged them to report any cases of harassment or intimidation as the row over Gordon Brown's behaviour towards officials paralysed the centre of government.
Labour's plans to build on its recent recovery in the polls – one of which shows today the Tories' lead narrowing to seven points – was thrown into crisis by the disclosure by the head of an anti-bullying charity that she had received worried calls from Downing Street staff.
But she was also thrown on the defensive yesterday by accusations from Labour that she was acting politically and charges from other charities that she had breached the confidentiality of bullying victims.
By last night four of the charity's five patrons, including the former Tory minister Ann Widdecombe, had resigned.
The turmoil was sparked by allegations on Sunday in a new book by the journalist Andrew Rawnsley about Mr Brown's intimidating behaviour towards staff.
Downing Street vehemently denied the main accusations and believed it had the story under control until the intervention of Christine Pratt, the founder of the National Bullying Helpline.
She disclosed she had taken "three or four" calls from Downing Street staff upset about their treatment. Her claims prompted a vitriolic counter-attack by Labour which accused her of acting politically.
Mr Brown's ministerial aide, Anne Snelgrove, said she had severed links with Mrs Pratt after receiving complaints that her helpline referred calls to her husband's human resources consultancy.
Lord Mandelson blamed the Tories for stoking up the row, while the former deputy prime minister John Prescott claimed she was attempting "a publicity stunt for her company".
Mrs Pratt strongly denied being driven by political motives.
But Labour was provided with ammunition in its attempts to undermine her credibility when she amended her version of events, saying her helpline had received "two [calls] from the Deputy Prime Minister's office and another two or more from the PM's office".
Mrs Pratt also acknowledged that none of the calls referred to claims of bullying by Mr Brown himself. Yesterday Downing Street initially refused to deny that Sir Gus O'Donnell, the Cabinet Secretary, had spoken to Mr Brown about his behaviour. Later Mr Brown's spokesman said: "The Cabinet Secretary would like to make clear that he has never raised concerns with the Prime Minister about him acting in a bullying or intimidatory manner in relation to No 10 staff, let alone giving him any sort of verbal warning."
The spokesman also said the number of complaints about bullying within the Cabinet Office, which includes Downing Street, was "very low" and below average for the Civil Service.
As the row raged, Mr Heywood, the Downing Street Permanent Secretary, sent an email to all staff, reminding them of the procedures in place if any of them should have concerns about bullying.
He wrote: "I don't believe for a moment that there is a culture of bullying and intimidation in No 10." Mr Heywood told them there was "a deep-rooted culture of teamwork, mutual support and friendliness" in Downing Street.
The spokesman explained that the email was "an encouragement" to staff, reminding them that there were procedures in place to deal with the issue.
The Conservatives last night said it was the "default position of the Downing Street machine to smear the messenger, as they have done so many times before".
David Cameron, the Tory leader, and Nick Clegg, the Liberal Democrat leader, both called for an inquiry into the bullying claims. Downing Street responded that there was no need for an investigation.
All parties are now closely watching the polls, the latest of which suggest the UK is on track for a hung parliament.
A survey by ICM for The Guardian showed the Tories' advantage narrowing to seven points. Labour has gained a point over the past month to reach 30 per cent, while the Conservatives were down three points on 37 per cent. The Liberal Democrats were down one point on 20 per cent.
If repeated at the general election, the findings would be likely to result in no party having an overall majority.
Mr Cameron said: "These are very serious matters. I'm sure No 10 Downing Street and the Civil Service in some way will want to have some sort of inquiry to get to the bottom of what has happened here."