The Metropolitan police are investigating how a newspaper managed to get hold of police transcripts detailing the incident between Andrew Mitchell and Downing Sreet officers, in which he is said to have called officers 'plebs'.
A Met police spokesperson told The Huffington Post UK they were looking into how police reports could have been seen by The Sun, which first reported the incident.
"We are aware of this and the directorate of professional standards has been informed. Inquiries into the circumstances are being carried out," they said.
The move comes after Downing Street backed Mitchell, who blamed a "long and frustrating day" for his outburst at officers.
The Sunsays transcripts of the incident, made by members of Scotland Yard's specialist protection team, show he called police "plebs" - a report backed up by at least two officers making the same verbatim note of the exchange in their pocket books.
According to the paper Mitchell called the police "morons" as well as "plebs" after they stopped him from leaving a Downing Street gate with his bike.
"Best you learn your f****** place. You don't run this f****** Government. You're f****** plebs," it reported him as saying.
The chief whip has denied calling police "plebs", however he refused to be drawn on what he actually said to the officer as he was questioned by reporters outside Whitehall on Monday.
"I have apologised to the police, I have apologised to the police officer involved on the gate and he's accepted my apology and I hope very much that we can draw a line under it there," he said.
Chief Superintendent Irene Curtis, president-elect of the Police Superintendents' Association, said it was still unclear whether Mitchell used the word "pleb".
She said: "We are five days on from when this incident happened and we're still no clearer as to what was said.
"By saying he disputes what the officer has said, he's actually calling into question the police officer's integrity, and for a police officer that's a very serious matter."
Ms Curtis added: "If he (Mr Mitchell) came out and said what he actually had said so we can understand why the officer came to the conclusion he did, then we can be a lot clearer."
Liberal Democrat Jeremy Browne, a Home Office minister, told the BBC: "I think people want to know what was said.
"Explaining to the media what was not said is not the same as explaining to the media what was said."