Chief inspector of constabulary Sir Thomas Winsor said police had an “enduring” duty of confidentiality, even after they had left the service.

He said if a serving officer had breached that duty they would face disciplinary action, potentially leading to dismissal and even criminal charges.

Sir Thomas said: “The special powers which citizens confer on police officers are inseparable from the obligations of special trust placed in police officers to enable them to do their duty. That trust requires every police officer to respect and keep confidential information which they obtain in the course of their duties and which is irrelevant to their inquiries and discloses no criminal conduct.

“The obligation of confidentiality, and the duty not to break trust, is an enduring one. It does not end when a police officer retires.”

His comments came after former Greater Manchester chief constable Sir Peter Fahy insisted that the men had a duty to keep information found during an investigation confidential, even after leaving the force.

But Sir Peter said: “It is very dangerous territory for a police officer to be making judgments about whether a politician is lying or not.

“Police should also be extremely careful about making judgments about other people’s morality when it is not a matter of crime.”

The row has been fuelled after former Scotland Yard detective Neil Lewis told the BBC he was “shocked” at the number of pornographic images uncovered in a 2008 raid on Mr Green’s Westminster office.

Mr Lewis said he was speaking out in support of former Metropolitan Police assistant commissioner Bob Quick, who went public with claims last month that pornography was found during an investigation into Home Office leaks.

Mr Green, who is currently the subject of a Cabinet Office inquiry, has denied looking at pornography on his work computer.