Andrew Mitchell could face an investigation by a parliamentary watchdog after his swearing-filled outburst against police officers outside No 10.
Just as the Tory chief whip thought he might have drawn a line under the row with a second apology this morning, he discovered Labour MP Bill Esterson had written to the parliamentary commissioner for standards asking him to look into the episode.
"David Cameron is showing himself too weak to sort this out," he said.
"He is refusing to detail what happened and refusing to ask for the Cabinet secretary to investigate.
"It is now about whether a government minister can get away with questioning the integrity of the police."
Although Mitchell accepts he lost his temper, he insists he did not use the phrases the Sun quoted him as saying – including that he called the police "morons" and "f****** plebs".
"It had been the end of a long and extremely frustrating day, not that that is any excuse at all for what happened," he told journalists outside the Cabinet Office this morning.
"I didn't show the police the amount of respect I should have done. We should all respect the police. They do an incredibly difficult job. I've apologised to the police, I've apologised to the police officer involved on the gate and he's accepted my apology. I hope very much we can draw a line under it there."
The Metropolitan Police Federation's chairman John Tully has called for Mitchell's resignation, however. Shadow home secretary Yvette Cooper has written to Cabinet secretary Sir Jeremy Heywood demanding to know what was really said.
Mitchell may have been hoist by his own petard. The police officer was reportedly happy to let the matter lie but felt it necessary to file a report on what was said after the chief whip said he had "not heard the last of this".
"Perhaps with the full verbatim report in the public domain, people can make up their own minds," a police source told the tabloid.
The Met and Mitchell, who works in No 9 when parliament is not sitting, are now at odds. When asked whether he used the word 'pleb', all Mitchell would say is: "I'm very clear about what I said and what I didn't say, and I want to make it absolutely clear I do not use the words that have been attributed to me."
The comment is constructed in such a way that it could refer to the precise quotes in the Sun, rather than the word 'pleb' in particular.