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The acting commissioner insisted that officers had received written permission from the Serjeant-at-Arms before searching the office of Mr Green.

He said that the police had acted without political prejudice in following the trail of evidence in their investigation into the leaking of private documents.

“It is our duty to follow the evidence, wherever that takes us,” he said. “The written authority to conduct a consensual search of those offices was obtained from the Serjeant-at-Arms at 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.”

He said that a file had been passed to the Crown Prosecution Service to consider whether there should be any prosecutions and that his officers must follow the evidence "without fear or favour" wherever it take them.

He said he wanted to take the opportunity to address some issues about the ongoing investigation of alleged leaks from the Home Office and that it was "right and proper for me to present some facts".

He said the controversial inquiry has “generated a great deal of publicity and important debate” but he suggested some was ill-informed.

“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.”

He said officers from the anti-terrorist branch was used solely because they had been merged with Special Branch officers whose responsibilities include official leaks. He also denied that any electronic listening equipment had been used during the investigation.

After initial inquiries in October he said that the CPS were consulted and officers believed that they had "reasonable ground to make an arrest".

He outlined the arrest of Chris Galley, a junior civil servant in November and said that a "number of items" were seized during a search.

He was interviewed by officers and bailed to appear again early next year. Two days later Mr Galley voluntarily attended a police station and was arrested and interviewed again before being rebailed.

Later that day, on November 21, the CPS were again consulted.

“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.”

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