David Davis has warned Downing Street not to sack Damian Green as a result of a "wrongful attempt by former officers to do him down", sources close to the Brexit Secretary have told the BBC.
One said Mr Davis might be willing to resign over the issue, although another stressed no threat had been made.
It comes after a retired detective said "thousands" of legal porn images had been found on Mr Green's computer.
Mr Green says he never watched or downloaded pornography on the computer.
Mr Davis has thrown a "protective cloak" around the First Secretary of State, who is effectively Theresa May's second-in-command, a source told the BBC.
The Brexit Secretary feels he "has a dog in the fight" because Mr Green was his subordinate on the Conservative shadow home affairs team in 2008 when the material was allegedly discovered on Mr Green's computer, the BBC's deputy political editor John Pienaar said.
According to one source, Mr Davis might be prepared to contemplate resignation, if he felt Mr Green had been "mistreated".
Another close associate said no threat of resignation had been made but the Brexit's secretary's strong feeling had been conveyed to Downing Street "at the highest level".
It was made clear that the warning from Mr Davis did not apply to the possibility of Mr Green losing his job under any circumstances, says John Pienaar.
"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".
Meanwhile, informed sources have suggested that a Cabinet Office report into Mr Green's conduct – which ranges wider than allegations of viewing pornography on an office computer, and covers accusations of inappropriate conduct – could be placed on the prime minister's desk early next week.
Former Scotland Yard detective Neil Lewis told BBC News "thousands" of thumbnail images of legal pornography had been found on Mr Green's computer during a 2008 inquiry into government leaks.
Conservative MP Andrew Mitchell defended Mr Green on BBC Radio 4's Today programme, saying: "It is the misuse of entirely legal information to blacken the name of a serving cabinet minister."
But Mr Lewis said a check of the computer's internet history over a three-month period showed pornography had been viewed "extensively" and a number of factors meant that he was sure it was Mr Green, the MP for Ashford, Kent, who was accessing the material.
Asked about the allegations in his constituency on Friday morning, Mr Green said: "I have maintained all along – I still maintain, it is the truth, that I didn't download or look at pornography on my computer but obviously while the investigation is going on, I can't say any more than that."
On Tuesday, Scotland Yard confirmed its department for professional standards was examining allegations that Mr Lewis had disclosed confidential information.
A statement from the Metropolitan Police said: "Confidential information gathered during a police inquiry should not be made public."
The Cabinet Office investigation into Mr Green was prompted by allegations of inappropriate behaviour in 2015 and 2016 made by Conservative activist Kate Maltby, which the MP has described as "completely false".
The inquiry is believed to centre on the ministerial code, which sets out the standards of conduct expected of government ministers.