Britain's most senior police officer has promised a "ruthless" investigation into an alleged conspiracy against a Cabinet minister "no matter where the truth takes us".
Met Police Commissioner Bernard Hogan-Howe defended his handling of the Andrew Mitchell "plebgate" row after breaking off from his holiday to be briefed on progress.
Mr Mitchell increased pressure on the force when he claimed he was the victim of a deliberate attempt to "toxify" the Tories and ruin his career.
The former chief whip was forced to quit his Cabinet post amid a storm of protest - fuelled by the Police Federation - over claims he called officers "plebs" during an altercation in Downing Street.
But last week Scotland Yard opened an investigation into a possible conspiracy against the MP after it emerged an email from a civilian witness backing up the claims was in fact written by another officer.
In a statement, Mr Hogan-Howe said: "The allegations in relation to this case are extremely serious.
"For the avoidance of doubt, I am determined there will be a ruthless search for the truth - no matter where the truth takes us."
He said the force's determination to get to the truth was proved by his decision to devote 30 officers to the task and the arrest of a member of the diplomatic protection squad and a civilian.
"I believe these actions are vital in maintaining public confidence in the police," he said.
Mr Hogan-Howe called for the investigation to be allowed "time and space".