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.
Thousands expected to take part in 'Derry Day' this weekend
Two new arrivals in DUP camp as UUP councillor Alan Lewis defects alongside serial party-switcher Henry Reilly
Brexit: There’s a fundamental con trick being played over Liz Truss’ Northern Ireland Protocol Bill says Lord Empey
BBC political editor Enda McClafferty sees the funny side after he’s caught on camera underdressed for live TV report
Rishi Sunak’s treasury ‘no friend of ours in fight against Protocol’: Paisley
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.”
But friends of Mr Green told the Press Association they were “gobsmacked” and found it “deeply concerning that a former police officer who freely admits talking to Bob Quick is putting confidential and non-illegal details of a police investigation into the public domain and equally outraged that the BBC would use such information from an unreliable source”.
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’.”
He also played down the possibility of the computer having been hacked, saying: “I can’t imagine that. It would be a very bizarre situation for somebody to hack a parliamentary computer to place pornography. There was also pornography internet history found on a separate machine, a laptop, of a similar nature.”
And he rejected outright any suggestion that Mr Green’s computer may have been mixed up with another one during the investigation: “Absolutely no way. I was the one who seized it from Portcullis House. I was the one who examined it, photographed it. I deal with computer forensics, that’s what I do. I produce evidence to court in relation to terrorists at the Old Bailey. It’s one of our key things, contamination.”
Mr Lewis said he did not mention the pornography in his formal statement on his findings to his senior investigating officer, as it had “no bearing on the leak investigation”.
But he said he had kept his notebook after leaving the Metropolitan Police because he felt “uncomfortable” about Operation Miser.
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.
Mr Benn told BBC Radio 4’s Today programme: “There is an inquiry going on, we are all awaiting the outcome of it, and clearly it should look at all available evidence before reaching a decision.
“I think all evidence that is relevant to the inquiry should be considered by the Cabinet Office. That’s what I would expect them to do.”
Mr Benn said he did not believe Mr Green should step down from his ministerial role pending the result of the inquiry, telling Today: “I think, in these circumstances, one should wait for the outcome of the inquiry. There’s a process and we should let it do its work.”
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.”
Mr Mitchell said the former detective had made clear that there was “no prosecutable evidence” obtained from the computer.
“Now, nine years later, after a pretty contentious raid of a senior politician’s office, entirely legal information is leaked to blacken the name of a serving Cabinet minister, and I think that is wrong,” said the Sutton Coldfield MP.
“I think it is highly questionable whether a retired police officer should misuse this sort of material in this way and I think the police need to explain why there was any record kept of entirely legal activity.
“He says he’s doing it to back up his friend Bob Quick ... and I don’t think that it is proper for retired police officers to behave in this way. I don’t think it’s giving Mr Green natural justice and I think it is wrong.”
Mr Mitchell said Mr Green should “certainly not” consider standing down.