Cabinet minister Damian Green is facing fresh claims that he viewed “thousands” of pornographic images on a Parliamentary computer.

Green was under huge political pressure after a former Scotland Yard detective told BBC News he was “shocked” by the amount of material found on a desktop device seized from the Tory MP’s Commons office.

Neil Lewis, who has not spoken publicly before, examined the computer during a 2008 inquiry into government leaks and was “in no doubt whatsoever” that Green had accessed legal pornography “extensively”.

A Cabinet Office inquiry is already underway into Green’s alleged viewing of porn, as well as claims that he behaved inappropriately towards a young Tory activist.

Green, who is First Secretary of State and effectively Theresa May’s deputy in the Government, has said he never watched or downloaded pornography at work and strongly denies any sexual harassment.

Friends of the minister told HuffPost UK that he was “gobsmacked” by the claims. “It is deeply concerning that a former police officer is, nine years on, putting into the public domain these smears of accessing non-illegal pornography,” one said.

But the Cabinet Office probe, led by Propriety and Ethics chief Sue Gray, will now face demands to interview Lewis as part of its inquiry.

Some Whitehall sources claim that the investigation has been completed after four weeks of evidence gathering, but No.10 has denied that any report has been sent to the PM.

After two Cabinet resignations last month, May will be reluctant to lose another even more senior minister.

Codes of conduct for MPs and ministers set out that they are expected to maintain “the highest standards of propriety”.

The pornography allegations were first alluded to by Bob Quick, a former Metropolitan Police Assistant Commissioner, in his evidence to the Leveson inquiry and resurfaced in the Sunday Times last month.

At the time Green responded to Quick’s assertions by accusing him of spreading “disreputable political smears”, an attack that so infuriated Lewis that he decided to speak out.

Former Metropolitan Police detective Lewis told the BBC that his check of the computer found in Green’s office revealed that pornography had been viewed extensively over a three-month period.

On some days, websites containing pornography were being searched for and opened for several hours, he said.

Lewis, who retired in 2014, said although “you can’t put fingers on a keyboard”, a number of factors meant that he was sure it was Green who was accessing the pornographic material.

His analysis of the way the computer had been used left the former detective constable in “no doubt whatsoever” that it was Green.

“The computer was in Mr Green’s office, on his desk, logged in, his account, his name,” said Mr Lewis, who at the time was working as a computer forensics examiner for SO13, the counter-terrorism command.

“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 Green’s laptop, he claimed.

Lewis disputed reports that the material found had been “extreme”. But he said that the Parliamentary authorities should have been informed about the “extensive” time Green allegedly spent looking at pornographic material.

“If a police officer does that, or anyone else, you’d be dismissed, you’d be thrown out.”

A spokesperson for Green 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.”

One friend added that it was unacceptable that former police officers were breaching their duty of confidentiality. At the time the computer was seized, Green was Shadow Immigration Minister and taking on the Labour government over ID cards and other big political issues. “It’s ridiculous to suggest he had time to do this kind of thing”.

Despite being told about Lewis’s role examining Green’s computers, the Cabinet Office inquiry has not contacted him to give evidence.