Rupert Murdoch's News International has reportedly handed over to police a data file containing hundreds of millions of its emails, as part of ongoing phone and email hacking investigations.

The data file handed to police is known as "Data Pool 3",  according to the Guardian newspaper, and is so large that police have decided they cannot effectively read every email. Instead, investigators are searching for specific keywords around hacking.

Last week, Rupert Murdoch travelled to the UK for meetings at News International, and visited its IT department. The reasons for the meetings have not been disclosed.

Access to News International's email archive has been a major issue during the hacking scandal. During 2009 and 2010, internal orders were issued at News International, for a pre-arranged email deletion plan to be executed, according to the Guardian - though it is not clear who made the orders. The instructions were reiterated several times over the following months but were not initially adhered to. This period also saw a renewed police investigation into the hacking scandal as victims, including Sienna Miller, made damages claims against the publisher.

The initial requests for email deletion may have been part of normal data management procedure, the newspaper reported. But last week, a High Court judge said News International had wilfully deleted emails that could have shown its journalists' participation in phone and computer hacking. As the company paid large settlement sums to a number of individuals over alleged hacking, the judge ordered a forensic search of the company's computers.

News International, which is facing growing accusations that several of its titles might be involved in hacking, has not commented. Two current and two former journalists on The Sun were arrested on Saturday, but have not been charged.

Legal experts told Computerworld UK that companies are within their legal right to delete emails as a way of controlling data volumes. However, they said, such deletion during a time of police investigations and court cases would be viewed highly unfavourably by a judge and could be a serious legal breach.

News International previously told the government's culture committee in carefully worded language that "technological corruptions" of its database had "resulted in some data being lost", and allegedly told victims' lawyers that it ditched some computers as part of an office move in 2010. It has also claimed, then later retracted the statement, that a raft of emails were lost en route to Mumbai.

Facing heightened legal pressure, the Guardian reported, Rupert Murdoch, chairman of parent group News Corp., has ordered all of the emails in Data Pool 3 to be handed over to authorities. It is understood that News International is also rapidly searching the database itself, in order to assess the information it contains. It has consistently said it is co-operating with the police.