A 52-year-old man has been questioned by detectives investigating claims that private investigators working for News International were involved in computer hacking.
The suspect was held on suspicion of Computer Misuse Act offences after being arrested in Milton Keynes, Scotland Yard said. He was later bailed to return to a London police station in early December.
The arrest came under the force's Operation Tuleta, an investigation into computer hacking, which runs alongside the Operation Weeting phone-hacking probe.
A Metropolitan Police spokesman said the man remains in custody at a Thames Valley Police station.
"Operation Tuleta is investigating a number of allegations regarding breach of privacy, received by the Metropolitan Police Service (MPS) since January 2011, which fell outside the remit of Operation Weeting, including computer hacking," the spokesman added.
Tuleta was launched over the summer after a "scoping exercise" into allegations surrounding the use of private detectives.
Officers working on the inquiry have been reporting to Deputy Assistant Commissioner Sue Akers, who heads the inquiry into mobile phone interceptions at the now defunct News of the World.
Scotland Yard's phone-hacking squad is working its way through 300 million emails from News International.
Commissioner Bernard Hogan-Howe says police have already spent up to £3 million on salaries, with officers speaking to 1,800 of 6,000 potential victims. A total of 120 officers and staff are now working on the entire investigation after 1,800 people came forward to express fears that they may have been hacked.
Jamie Pyatt, 48, became the first Sun journalist to be arrested earlier this month as part of Operation Elveden, an investigation into illegal payments to police. Other suspects include former News International chief executive Rebekah Brooks, ex-Downing Street communications chief Andy Coulson and the Sunday tabloid's former royal editor Clive Goodman.