Ex-director of public prosecutions Lord Macdonald said it was "outrageous" the force had yet to publish a conclusion on the Downing Street incident, which led to then chief whip Andrew Mitchell's resignation.
The Metropolitan Police said the inquiry was complicated, while the Independent Police Complaints Commission (IPCC) said all that could be done was being done.
Deborah Glass, deputy chairwoman of the IPCC, which is supervising two investigations, said: "What has evolved are allegations of a conspiracy which by its very nature is complex."
Mr Mitchell was accused of calling officers "plebs" after being refused permission to cycle through the main gates. He denied the claims and said he was the victim of a bid to ruin his career.
A Met spokesman said a file was passed to prosecutors, but since then "three separate pieces of information have been given to us."
Nine Met officers have been or are being probed for criminal and misconduct offences, along with four civilians. Another probe surrounds claims a West Mercia Police Federation representative gave a false account of a meeting with Mr Mitchell.