Police are investigating evidence that a News International executive may have deleted millions of emails from an internal archive in an apparent attempt to obstruct Scotland Yard's inquiry into the phone-hacking scandal.

The archive is believed to have reached back to January 2005, revealing daily contact between News of the World editors, reporters and outsiders, including private investigators. The messages are potentially highly valuable both for the police and for the numerous public figures who are suing News International (NI).

According to legal sources close to the police inquiry, a senior executive is believed to have deleted "massive quantities" of the archive on two separate occasions, leaving only a fraction to be disclosed. One of the alleged deletions is said to have been made at the end of January, just as Scotland Yard was launching Operation Weeting, its new inquiry into the affair. The allegation directly contradicts NI claims that it is co-operating fully with police in order to expose its history of illegal newsgathering.

The alleged deletion of emails will be of particular interest to the media regulator Ofcom, which said it had asked to be "kept abreast" of developments in the Met's hacking investigation, so it can assess whether News Corp would pass the "fit and proper" test that all owners of UK television channels have to meet.

That came amid the first signs that Rebekah Brooks's grip on NI was weakening on a dramatic day when David Cameron all but called for her resignation. It also emerged that Brooks was no longer in charge of the company's three-person in-house standards committee that is tackling the hacking issue, and that Rupert Murdoch is planning to fly into London on Sasturday to confront the crisis.

Earlier on Friday, the prime minister told reporters at an emergency press conference on the crisis on Friday morning: "It's been reported that [Brooks] had offered her resignation, and in this situation I would have taken it." Although NI denies that she offered to resign, Cameron's meaning was clear as he tried to distance himself from a person with whom he has a close social relationship.

The scandal brought a number of arrests on Friday, with the prime minister's former PR chief Andy Coulson held under suspicion of involvement in phone hacking. As he was released on bail, he told reporters: "There is an awful lot I would like to say, but I can't at this time." Clive Goodman, the NoW's former royal reporter, was also arrested in relation to the alleged payment of bribes to police, and subsequently bailed. And last night an unnamed 63-year-old man was also arrested in connection with alleged corruption.

Cameron yesterday announced that his administration will launch a full judicial review into phone hacking defended his hiring of Coulson, and in effect announced that the Press Complaints Commission would be scrapped after 20 years and replaced by a new regulatory body independent of the newspaper industry and the government. He said all party leaders had been "so keen to win the support of newspapers that we turned a blind eye to the need to sort this issue".

The Guardian understands that the suspected deletion of emails is one of a number of actions that have infuriated detectives investigating hacking. In addition to deleting emails, NI executives have also:

• Leaked sensitive information in spite of an undertaking to police that they would keep it confidential.

• Risked prosecution for perverting the course of justice by trying to hide the contents of a senior reporter's desk after he was arrested earlier this year.

NI originally claimed the archive of emails did not exist. Last December, its Scottish editor, Bob Bird, told the trial of Tommy Sheridan in Glasgow that the emails had been lost en route to Mumbai. Also in December, the company's solicitor, Julian Pike from Farrer and Co, gave a statement to the high court saying it was unable to retrieve emails more than six months old.

The first hint that this was not true came in late January when NI handed Scotland Yard evidence that led to the immediate sacking of its news editor, Ian Edmondson, and to the launch of Operation Weeting. It was reported that this evidence consisted of three old emails.

Three months later, on 23 March, Pike formally apologised to the high court and acknowledged News International could locate emails as far back as 2005 and that no emails had been lost en route to Mumbai or anywhere else in India. In a signed statement seen by the Guardian, Pike said he had been misinformed by the NoW's in-house lawyer, Tom Crone, who had told him that he, too, had been misled. He offered no explanation for the misleading evidence given by Bird.

The archive was said to contain half a terabyte of data – equivalent to 500 editions of Encyclopaedia Britannica. But police now believe that there was an effort to substantially destroy it before NI handed over their new evidence in January. Police believe they have identified the executive responsibleby following an electronic audit trail. They have also attempted to retrieve the lost data. The Crown Prosecution Service is believed to have been asked whether the executive can be charged with perverting the course of justice.

At the heart of the affair is a data company, Essential Computing, based near Bristol. Staff there have been interviewed by Operation Weeting. One source speculated that this company had compelled NI to admit that the archive existed.

The Guardian understands that Essential Computing has co-operated with police and provided evidence about an alleged attempt by the NI executive to destroy part of the archive while they were working with it. This is said to have happened after the executive discovered that the company retained material of which NI was unaware.

The alleged deletion has caused tension between NI and Scotland Yard, who are also angry over leaks. When it handed over evidence of journalists' involvement in bribing officers, it wanted to make an announcement, claiming credit for its assistance to police. NI was warned that this would interfere with inquiries and finally agreed to keep the entire matter confidential until early August, to allow police to make arrests. In the event, a series of leaks this week has led Scotland Yard to conclude that NI breached the deal.

There was friction this year when police arrested a senior journalist. When they went to the NoW office to search his desk, they found its contents had been removed and lodged with a firm of solicitors, who refused to hand them over. They eventually complied. A file is believed to have been sent to the Crown Prosecution service seeking advice on whether anybody connected with the incident should be charged.