Ex-detectives who disclosed that legal pornography was found on Damian Green's office computer are in "dangerous territory", a former police chief says.
Sir Peter Fahy, ex-chief constable of Greater Manchester, said it was vital police were "not involved in politics".
And a watchdog warned that serving officers could be sacked or prosecuted for leaking confidential information.
First Secretary of State Mr Green denies watching or downloading pornography on his computer.
The allegations were first made last month by former Metropolitan Police assistant commissioner Bob Quick, who led a 2008 inquiry into Home Office leaks which saw Mr Green's Commons office being searched.
Mr Quick made his claims after the Cabinet Office launched an investigation into accusations of inappropriate behaviour by Mr Green towards journalist Kate Maltby, which the MP has described as "completely false".
And then on Friday, retired Met detective Neil Lewis said "thousands" of thumbnail images of legal pornography had been found on Mr Green's parliamentary computer in 2008.
Despite being told about Mr Lewis's role examining Mr Green's computers, the Cabinet Office inquiry has not contacted him to give evidence.
Sir Peter, Greater Manchester's chief constable for seven years, told BBC Radio 4's Today programme that the former officers had a duty to protect confidential information uncovered during an investigation.
He said: "It is very dangerous territory for a police officer to be making judgements about whether a politician is lying or not.
"That should only happen in a criminal investigation, and even then, ultimately it is for a court to decide."
Scotland Yard's department for professional standards is examining allegations that Mr Lewis had disclosed confidential information.
Her Majesty's Chief Inspector of Constabulary Sir Thomas Winsor said the police's special powers were inseparable from "obligations of special trust".
This trust required every 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", he said.
"If a police officer broke that trust whilst serving as a police officer, he or she would face disciplinary action and could be dismissed.
"In certain circumstances, such action could also constitute a criminal offence."
The obligation was enduring and "does not end" when a police officer retires, he said.
According to the College of Policing's Code of Ethics, which applies to forces in England and Wales, police agree to disclose information "only in the proper course of my duties".
Tory MPs, including Brexit Secretary David Davis, have backed Mr Green - effectively Theresa May's second-in-command - saying it was wrong for such claims to emerge through the media.
On Friday, sources close to David Davis told the BBC the Brexit Secretary had warned Downing Street not to sack Mr Green as a result of a "wrongful attempt by former officers to do him down".
One said Mr Davis might be willing to resign over the issue, although another stressed no threat had been made.
"It's right that allegations of misconduct towards individuals are properly investigated," said an associate of Mr Davis.
"But police officers have a duty of confidentiality which should be upheld".
Ex-attorney general Dominic Grieve said the two ex-police officer's actions were "very worrying" and smacked of the "police state".
Mr Grieve, attorney general in David Cameron's government between 2010 and 2014, described the decision by the officers to release the information from a police investigation dating back to 2008 as a "flagrant breach" of their code of conduct.
"Eight years later they choose to put material that an ordinary citizen would be prohibited from acquiring under data protection rules into the public domain on their own judgement," he told BBC Two's Newsnight.
"I find it very worrying.
"If you think something is relevant, you do it by proper official means.
"You do not go freelancing as these two officers have done...
"We give the police powers that other people do not have. They are not and must not be allowed to abuse those powers."
However, former Gloucestershire chief constable Tim Brain said that the information was in the public interest.
He told Newsnight that the officers had come forward with what they considered "relevant information" to an ongoing inquiry.
Mr Brain said: "Let's just think about this as a workplace computer and to think whether we are happy that people, our MPs, can have this kind of material on what is an official computer."
What is the row all about?
- The allegations date back to November 2008 when the Conservatives were in opposition and Damian Green was their immigration spokesman
- He embarrassed the Labour government with a series of leaks about illegal immigration and other issues
- Police raided his Parliamentary office as part of an inquiry into how he got his hands on confidential documents
- His office computer was seized and he was detained for nine hours - sparking fury among Tory MPs
- No charges were brought against the MP or civil servant Christopher Galley, who passed unauthorised material to Mr Green, although the official was sacked
- The officer in charge of the inquiry, Bob Quick - widely criticised by Tory MPs for the raid - resigned six months later over an alleged security breach
- But a month ago, Mr Quick disclosed that pornographic material had been found on the seized computer, adding that the police had not pursued the matter at the time or told Mr Green about it
- The matter has since been investigated by the Cabinet Office as part of a wider inquiry into claims of inappropriate behaviour by Mr Green towards journalist Kate Maltby
- The inquiry is believed to centre on the ministerial code, which sets out the standards of conduct expected of government ministers.
Other MPs to criticise the action of the former officers included the chairman of the Commons culture committee, Damian Collins.
He told BBC Radio 4's Any Questions the Cabinet Office "should receive and see any evidence" that's pertinent to its investigation.
The DUP's leader at Westminster, Nigel Dodds, said "police are not above the law" and their actions had been "totally wrong".
Speaking to reporters at his Kent home on Friday, Mr Green said: "I have maintained all along and I still maintain - it is the truth - that I did not download or look at pornography on my computer, but obviously while the investigation is going on, I can't say any more than that."