Scotland Yard is investigating allegations of fraud involving a "substantial sum" of money belonging to the Prison Officers' Association.
The Fraud Squad investigation, which began last March, was revealed yesterday as the POA held a special conference to discuss allegations of expenses irregularities and poor financial controls.
Scotland Yard said inquiries were ongoing but that there had been no arrests at this stage.
A spokeswoman for the POA, which had hired auditors to carry out its own investigation, said the association had "not been aware" it was being investigated by the police. Its investigations are believed to centre on the size of travel and entertainment bills and the over-use of union credit cards.
She said she could not reveal the exact amount of money involved, but that it was "not telephone numbers".
John Bartell, former chairman of the POA, confirmed yesterday that he had been sent a 50-page questionnaire by the union about expenses going back over the past three years.
Mr Bartell, who retired on health grounds last July, maintained that no allegations had been made against him and he was confident that the union's accounts had been properly audited during his chairmanship. He attacked the conference as a "kangaroo court" and said he feared delegates at the London conference would be dispensing "mob justice" without knowing the full facts.
Three hundred delegates representing the union's 29,000 members in England, Wales and Northern Ireland are attending the conference, which concludes today.
The union's recently appointed national chairman, John Boddington, who does not face any allegations of impropriety himself, yesterday conceded that the mood of his members was one of "anger and frustration", andsaid that he had called the conference to reassure members. "The financial controls are now improved - we have put the house in order," he said.Reuse content