Chief Whip Andrew Mitchell failed to silence calls for his resignation after a meeting with police did nothing to resolve the dispute over his foul-mouthed rant at officers.
The chairman of the West Mercia Police Federation, Ken Mackaill, said the politician had "no option but to resign" after refusing to give details of exactly what he said in his tirade.
"He is continuing to refuse to elaborate on what happened and I think his position is untenable," Mr Mackaill said as he emerged from the 45-minute meeting in Mr Mitchell's Sutton Coldfield constituency.
While he had given a "profound apology" for his altercation over a refusal to allow him to cycle through Downing Street's main gate, Mr Mitchell continued to dispute the officers' report that he called them "plebs", Mr Mackaill said.
"He refused to tell us what he did say on the grounds he did not want to impugn police officers' integrity and start, I quote the word, a 'firefight' with police. Unfortunately that is exactly what happened: the question of integrity remains unresolved."
Mr Mackaill, who was accompanied at the meeting by the secretaries of the West Midlands and Warwickshire federations, said integrity was "a massive issue" among rank and file officers.
"This is about the honesty and accuracy of police records and there are implications for officers giving evidence in court, after all. We take the view that this is a Cabinet minister challenging the accuracy of police records and that, we think, is of interest to all police officers."
Mr Mitchell left without making comment amid pressure on Prime Minister David Cameron to sack him - just weeks after appointing him to the key position in the reshuffle.
The Cabinet member did not attend this week's Conservative Party conference amid speculation that he could yet lose his job, weeks after the initial report in The Sun. But that failed to shift attention from the issue.
Shadow home secretary Yvette Cooper said the time had passed for clarifications and apologies over Mr Mitchell's alleged use of the word "pleb", and said Mr Cameron had to act.