Police investigating computer hacking by private investigators commissioned by national newspapers have uncovered evidence that emails sent and received by Gordon Brown during his time as Chancellor were illegally accessed.

Mr Brown's private communications, along with emails belonging to a former Labour adviser and lobbyist, Derek Draper, have been identified by Scotland Yard's Operation Tuleta team as potentially hacked material. They are currently looking at evidence from around 20 computers which hold data revealing that hundreds of individuals may have had their private emails hacked.

The links discovered from the seized computers suggest that the email investigation could involve as many victims as those involved in the News of the World phone-hacking scandal.

The eight-strong Tuleta team is looking at the possibility that several Fleet Street titles commissioned specialist private detectives to access computers. News International yesterday declined to comment on the latest allegations.

A source with knowledge of the contents of some of the computers seized from private investigators told The Independent that analysis of a portion of the hundreds of thousands of messages found on the machines showed that Mr Brown and Mr Draper were targeted while the former Prime Minister was Chancellor of the Exchequer. The period includes potentially sensitive episodes in the difficult relationship between Mr Brown and Tony Blair.

One of Mr Brown's former cabinet colleagues, Peter Hain, has confirmed that he held discussions with police officers investigating the potential hacking of his computers during the period when he was Northern Ireland Secretary.

The period discussed with Mr Hain, from 2005 to 2007, overlaps with the period Operation Tuleta is looking at in connection with the Brown-Draper emails. Scotland Yard last night declined to discuss its inquiry into the electronic eavesdropping. A spokesman said: "We are not prepared to give a running commentary on an ongoing investigation."

NI's chief executive, Tom Mockridge, said his company had been advised that Mr Hain's computer equipment "was not and has not been the subject of an investigation by Operation Tuleta" and that there was "no belief or suspicion that this equipment was hacked".

Mr Hain, however, said he had met with the head of Operation Tuleta, Detective Inspector Noel Beswick, and discussed the hacking of three of his computers: two issued by the Northern Ireland Office, and a personally owned machine. The Tuleta team has also interviewed a former Army intelligence officer who has made a formal complaint that his computer was illegally accessed six years ago as part of a search for documents associated with the province's Deputy First Minister, Martin McGuinness.

Mr Brown has previously accused News International of accessing parts of his private life including his bank accounts. He said he "could not understand" why he had the protection and defences of a chancellor or prime minister, and yet remained vulnerable to "unlawful or unscrupulous tactics".

Earlier this year Mr Brown sent Scotland Yard tape recordings which he claimed challenged NI assurances that The Sunday Times had broken no laws when it investigated his personal financial affairs. He told Sue Akers – the Met's Deputy Assistant Commissioner who is leading the phone-hacking and email-hacking investigations – that three senior Sunday Times journalists, whom he named, were aware of the "blagging" techniques used to access his personal details.

Mr Draper, a former lobbyist and former assistant to Lord Mandelson, has found his private correspondence being published on two occasions that have damaged the Labour Party and the reputation of Gordon Brown.

In 2008 a sequence of email exchanges between Mr Draper and Lord Mandelson damaged a planned make-over of Mr Brown's reputation during his difficult time as Prime Minister. In the leaked emails, Mr Brown was described as a "self-conscious person, physically and emotionally" and someone "not comfortable in his own skin". In 2009 leaked emails between Mr Draper and Gordon Brown's head of strategy and planning, Damian McBride, offered a series of planned smears targeted at David Cameron and George Osborne. It was suggested that the Tory leader could be falsely branded as having an embarrassing medical condition, and that Mr Osborne, then shadow Chancellor, could be alleged to have taken drugs with a prostitute. Although all the allegations were nonsense, Mr Draper, then re-emerging as a prominent pro-Labour blogger, wrote back to Mr McBride saying "Absolutely totally brilliant Damian."

There is no suggestion that any of this material was accessed through illegal computer hacking techniques.