The charity at the centre of the Gordon Brown bullying row has lost one of its patrons.
Professor Cary Cooper resigned from the National Bullying Helpline on Monday morning, after accusing the charity's boss of breaching confidentiality.
The stress expert wrote an email to Christine Pratt, saying: "One of the things that is really important for any helpline or any counselling service is to retain confidentiality of the people calling up."
The move came after Mrs Pratt entered the row over the Prime Minister's behaviour, saying the helpline had received several calls from Downing Street employees.
Her revelations sparked calls for an investigation into Mr Brown's treatment of staff.
Mrs Pratt said she went public with the information after ministers rallied round to deny suggestions in a new book that the Prime Minister had been warned over his behaviour.
Lord Mandelson said the Prime Minister could be emotional and demanding but was not a bully, following the claims that a string of verbal and physical outbursts had led to the intervention of the Cabinet Secretary.
But Mrs Pratt, who founded the helpline, accused ministers of failing staff by "going into denial".
She said: "We would have hoped Gordon Brown would lead by example. If an employer receives complaints they should investigate."
She added: "I have personally taken a call from staff in the Prime Minister's office, staff who believe they are working in a bullying culture and that it has caused them some stress."
"I am not saying Gordon Brown is a bully, I am not a judge. But I am appalled at the outright denial that is going on without due process being followed."
The book, by Observer political commentator Andrew Rawnsley, includes accounts of Mr Brown pulling a secretary from her chair, "roughly shoving" an aide and four-letter-word rants that frightened staff.
In extracts published yesterday by the newspaper Mr Rawnsley said Cabinet Secretary Sir Gus O'Donnell was so concerned he delivered a "verbal warning" to the PM - a claim the Cabinet Office said was "completely untrue".
However, Mrs Pratt's intervention has led to a bitter political war of words, after she admitted she had spoken to Conservative MPs about the claims.
Tory MP Anne Widdecombe is one of the charity's patrons. Mrs Pratt said other parliament insiders had called her to offer their support, but insisted the calls bore no relevance to her decision to go public.
She added: "There is no political edge at all. We are a non-political organisation."
However, Former Home Secretary Jacqui Smith confirmed that she wrote to the National Bullying Helpline yesterday, sending a copy to the charity's patron, Ann Widdecombe.
Ms Smith's letter read: "I am writing to express my absolute amazement that you have so comprehensively breached the commitment made on your website to treat calls as confidential and to respect the dignity of those who use your service.
"The public statements of your chief executive about receiving calls from members of the Prime Minister's Office drive a coach and horses through your confidentiality and impartiality policies.
"I have previously advised constituents facing difficult bullying situations about your service. I will never do that again!
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