The former aide of Commons Speaker Michael Martin has told MPs he feels "very sad" he had to quit last month, but said Mr Martin was not to blame.
Mr Granatt resigned over a story about Mr Martin's wife
Mike Granatt quit after he unwittingly misled a journalist about reports Mr Martin's wife claimed £4,000 taxi expenses since May 2004.
He has said officials did not tell him the whole truth.
He told MPs on Thursday: "There are people who should not have let it happen. That is betrayal in a sense."
But while he refused to name names, he told the Commons public administration committee he was not referring to Mr or Mrs Martin.
Mr Granatt, a former top Whitehall communications official who became Mr Martin's spokesman in May 2005 said he had felt he had undertaken "due diligence" with respect to the explanation about the taxi trips.
But he added: "The one mistake I made was in believing that the standards and behaviour of employees and staff in Whitehall over 25 years I would find elsewhere."
Mr Granatt, who is a senior partner at Luther and Pendragon public relations company, said he was not criticising Mr or Mrs Martin.
His resignation followed stories which appeared at the end of 2007, that Mrs Martin had claimed more than £4,000 in taxi expenses since May 2004.
He said the journeys had been shopping trips for official functions and that Mrs Martin had been accompanied by an official at all times, but he was later told by a journalist that the official was in fact a housekeeper working for the Speaker.
Mr Granatt said he was resigning for ethical reasons, after unwittingly misleading the journalist, but blamed officials for not telling him the entire truth.
Asked by MPs on the Public Administration Committee to say who had misled him, Mr Granatt said he was, "not prepared to go into details of names or places... It wasn't the Speaker and it wasn't, as it was put to me, Mrs Martin."
Asked if he felt "very bitter or betrayed" he replied that he felt, "very sad" adding: "Betrayal is a strong word and there are people who should have not let it happen and that is a betrayal in a sense."
Mr Martin is heading a "root and branch" parliamentary inquiry into all aspects of MPs' expenses, in the wake of a furore over Tory MP Derek Conway's employment of his sons.
Parliament's standards watchdog is currently investigating a complaint over Mr Martin's use of air miles gained from official trips to fly members of his family from Glasgow to London for a New Year break.
Mr Granatt, widely regarded as an expert in communications with a long career in the civil service, joined public relations consultancy Luther Pendragon in 2004.
The House of Commons Commission, the management committee which Mr Martin chairs, was one of his many clients.