Gordon Brown's treatment of his staff has come under further scrutiny after an anti-bullying charity said several Downing Street employees had called its helpline seeking advice and counselling.

Christine Pratt, the chief executive of the National Bullying Helpline, said that the charity had been called three or four times in recent months by members of Brown's office over his behaviour.

"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," she said. "We would have hoped Gordon Brown would lead by example. If an employer receives complaints, they should investigate."

She added that she spoke out in anger at No 10's "outright denial" of claims that the prime minister bullied and abused staff.

It was reported on Sunday that Sir Gus O'Donnell, the head of the civil service, had warned Brown about his treatment of personnel. The allegation is contained in a new book by the Observer's chief political commentator Andrew Rawnsley. The End of the Party reports that O'Donnell intervened informally to "calm down frightened duty clerks, badly-treated phone operators and other bruised staff by telling them: 'Don't take it personally'."

Labour MP Anne Snelgrove, who is Brown's parliamentary private secretary, said the charity "needs to demonstrate that these questions really have come from staff at Number 10".

"Why is she [Ms Pratt] going public with this rather than taking it up privately if these phone calls were genuine?"

Downing Street said it had never been contacted by the charity about the allegations, a No 10 spokesman adding that it had "rigorous, well established procedures" for "staff to address any concerns over inappropriate treatment or behaviour".

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