Sir Hugh Orde, president of the Association of Chief Police Officers, said the officers accused of conspiring against former Cabinet minister Andrew Mitchell had fallen below the standard expected.
However, Sir Hugh told BBC Radio 4's Today programme that their chief constables should be given the opportunity to explain their decision not to take further disciplinary action.
He said: "It requires an explanation from the leaders of those three police forces, all of whom have very clearly said that they want to come to the Home Affairs Committee and be held to account in an open and transparent forum to explain their side of the story which is currently running.
"The question is the quantum of seriousness and I think that's why the chief constables are clearly determined to explain that ... to the Home Affairs Select Committee and be held to account and judged on that."
It's turning out to be a big week in the life of the story we call "plebgate".
West Mercia Police have confirmed they are reviewing a report on the 'Plebgate' scandal which is alleged to have been fundamentally altered.
Claims have emerged police officers changed the conclusions of a probe into a meeting between Andrew Mitchell and federation members.