A mole hunt into a number of embarrassing leaks has resulted in the arrest of a civil servant and a Conservative frontbencher.

Shadow immigration minister Damian Green was arrested on Thursday and questioned for nine hours by the Metropolitan Police.

He was held on “suspicion of conspiring to commit misconduct in a public office and aiding and abetting, counselling or procuring misconduct in a public office” before being released without charge on unconditional bail, a police spokesman said.

It follows the arrest of a junior official from the Home Office last week “in connection with an inquiry into alleged leaked documents”, according to a spokesman for the department.

Senior civil servants are thought to have ordered an internal investigation after a number of stories were released to the press.

They included news that 5,000 illegal immigrants were working as security guards and bouncers; the revelation that an illegal immigrant was employed as a cleaner in the House of Commons; a whip's list of potential Labour rebels against 42-day detention for terror suspects; and a letter from home secretary Jacqui Smith to the prime minister warning that the recession would spark a rise in crime.

Green has denied any wrong doing and he and his colleagues have expressed anger at the way the matter was handled.

Conservative leader David Cameron described it as “extraordinary” and “heavy-handed” and said government ministers had some serious questions to answer.

The prime minister, home secretary and the police have all denied that anyone in the government had prior knowledge of the police’s decision to arrest Green.

Gordon Bown said he had only been told about it by cabinet secretary Sir Gus O’Donnell after the event.

“As I said to him, this is a matter for the police, they are carrying out an investigation,” Brown said. “It is not a matter for government ministers.”