"Thousands" of legal pornographic images were found on a computer used by First Secretary of State Damian Green during a police raid in 2008, a retired Scotland Yard detective has said.
Neil Lewis told the BBC he was "shocked" at the volume of material and had "no doubt whatsoever" that it was accessed by the Tory MP.
He stressed that none of the images were "extreme", but said analysis of the computer suggested they had been viewed "extensively" over a three-month period, sometimes for hours at a time.
Mr Green, who is the subject of a Cabinet Office inquiry into alleged inappropriate behaviour towards a young female activist, has denied looking at or downloading porn on the work computer.
The minister - effectively Theresa May's deputy - declined to comment on Mr Lewis's allegations.
But friends of Mr Green said they were "gobsmacked" at the former detective putting his claims into the public arena and "outraged" by the BBC's decision to broadcast them.
Mr Lewis told the BBC he was involved in analysing the then opposition immigration spokesman's computer during a police investigation into Home Office leaks.
He said that although "you can't put fingers on a keyboard", a number of factors meant that he was sure it was Mr Green himself who was accessing the "thumbnail" images.
"The computer was in Mr Green's office, on his desk, logged in, his account, his name," said Mr Lewis. "In between browsing pornography, he was sending emails from his account, his personal account, reading documents ... it was ridiculous to suggest anybody else could have done it."
Similar material had also been accessed on Mr Green's laptop, he claimed.
The allegations echo claims made by former police chief Bob Quick, whom Mr Green branded "tainted and untrustworthy" after he went public last month with his account of the material discovered in the raid.
A spokesman for the First Secretary of State said: "It would be inappropriate for Mr Green to comment on these allegations while the Cabinet Office investigation is ongoing; however, from the outset he has been very clear that he never watched or downloaded pornography on the computers seized from his office.
"He maintains his innocence of these charges and awaits the outcome of the investigation."
Recalling the raid carried out on the Ashford MP's office in Westminster's Portcullis House office block as part of an operation codenamed Miser, Mr Lewis told the BBC: "The shocking thing, as I was viewing, I noticed a lot of pornography - thumbnails - which indicated web browsing.
"There were a lot of them. I was surprised to see that on a parliamentary computer. I had to take a step back, because I wasn't expecting that. When you ask me a number, I couldn't tell you. There were thousands."
Asked whether it was possible that someone else in Mr Green's office had been viewing porn on his computer, Mr Lewis said: "It was so extensive, whoever had done it would have to have pushed Mr Green to one side to say 'Get out, I'm using your computer'."
Asked why he was making his claims public now, Mr Lewis said: "In the last few weeks, there was an article in the Sunday Times in relation to Damian Green having pornography on his computers. His outright denial of that was quite amazing, followed by his criticism of Bob Quick, my senior officer.
"So I contacted Bob Quick to offer my support, really. When I left the police, I kept one notebook and that was the notebook for Operation Miser, because that was the case that I was uncomfortable with. All the others - paedophiles and terrorists - they are past and gone. This one case, this one, Operation Miser, I've never been comfortable with."
Senior Labour MP Hilary Benn said Mr Lewis's allegations should be considered by the Cabinet Office inquiry into Mr Green's conduct.
Conservative former minister Andrew Mitchell, a friend of Mr Green, told Today: "Mr Green has been absolutely emphatic in what he said. He has said repeatedly that he never downloaded or viewed this material.
"I think Mr Green is entitled to be believed. After all, you are not guilty until proven so in this country and I think the hounding of Mr Green over information which everyone is clear was entirely legal and which he has emphatically denied either downloading or viewing is completely wrong."