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 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."

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."

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 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."

Mr Lewis told the BBC that he was instructed by the Metropolitan Police to delete data from copies he had made of information on Mr Green's computers.

"Morally and ethically I didn't think that was a correct way to continue," he said.

He added that he erased the data as requested but kept the copies in the knowledge that experts would later be able to retrieve the information if required. However, he now believes the items may have been destroyed.

Scotland Yard said in a statement: "Confidential information gathered during a police inquiry should not be made public.

"The appropriate course of action is to co-operate privately with the Cabinet Office inquiry as the [Metropolitan Police] has done.

"As is routine for cases of this nature the circumstances of information being made public will be looked at by the Directorate of Professional Standards."