Scotland Yard's acting chief launched an outspoken defence of his officers today in the wake of the arrest of MP Damian Green.
Acting Commissioner Sir Paul Stephenson said detectives in any inquiry, no matter how sensitive, must follow the evidence "without fear or favour".
Speaking at City Hall in London, he said: "Clearly this was going to be a sensitive investigation and it is right we should be held to account at the appropriate time."
He added: "At issue in this investigation, and the work of the service as a whole, is our ability to maintain operational independence.
"The police must be able to act without fear or favour in any investigation, whomsoever may be involved, where there are reasonable grounds to suspect they may have committed criminal offences."
Responding to claims that the police had been influenced by senior Cabinet figures, Sir Paul said: "I would strongly refute that I or any senior officer under my command have or have allowed any improper influence of our operations or acted for political purposes. That is not what we do."
In his first public appearance since taking over the force, Sir Paul said he wanted to set the record straight over the investigation into Mr Green.
He said the controversial inquiry has "generated a great deal of publicity and important debate" but he suggested some was ill-informed.
He said: "There is a large amount of comment and speculation already in the public domain and I think it is right and proper to present some facts surrounding this investigation which lie at the heart of how the Metropolitan Police operates."
Sir Paul outlined the investigation conducted since the arrest of a junior civil servant on 19 November by officers from Counter Terrorism Command.
He said officers from this branch of the force were involved because they include former Special Branch officers whose responsibilities include official leaks.
Sir Paul said: "It is our duty to follow the evidence wherever that may take us. It was as a result of the initial investigation and arrest that the decision was made by officers under the command of Assistant Commissioner Bob Quick to arrest an MP and to search three addresses connected with him under authority of warrants."
Sir Paul said permission to search the MP's parliamentary office was obtained from the Serjeant-at-Arms of the Palace of Westminster.
He said: "Officers have an obligation to look and secure evidence to avoid any circumstances where potential evidence could be lost.
"With this in mind, the decision was taken to arrest and search the relevant addresses in accordance with the Police and Criminal Evidence Act."
Sir Paul said an initial report has been passed to officials at the Crown Prosecution Service (CPS) for guidance on the next steps of the inquiry.
He said: "The public interest in this case means there has been inevitably a large amount of speculation and comment around our actions."
Sir Paul denied claims that Met officers attempted to use the civil servant to trap Mr Green or employed electronic listening devices to monitor the MP.
Sir Paul's statement comes after he announced yesterday that Ian Johnston, chief constable of British Transport Police, has been appointed to review the handling of the police inquiry.
The decision reflected Scotland Yard's concern about the mounting political furore over Mr Green's arrest and the search of his office in the House of Commons.
The Ashford MP, who denies all wrongdoing, has been bailed until February.
Civil servant Christopher Galley, 26, who is accused of passing him sensitive Home Office documents, has also been arrested and bailed.