Earlier this week the Conservative opposition party immigration spokesman Damian Green was arrested and released on bail in connection with a series of leaks from the Home Office.
Police say Mr Green was held on suspicion of “conspiring to commit misconduct in a public office”.
The MP denied any wrongdoing and said “opposition politicians have a duty to hold the government to account” and that he would “continue to do so”.
He was questioned, but has not been charged and was bailed until February.
Mr Green’s arrest is believed to be connected to the arrest of a man suspected of being a Home Office whistleblower.
Speaking outside the House of Commons, Mr Green said: “I was astonished to have spent more than nine hours today under arrest for doing my job.
“I emphatically deny I have done anything wrong. I have many times made public information that the government wanted to keep secret – information that the public has a right to know.
“In a democracy, opposition politicians have a duty to hold the government to account.
“I was elected to the House of Commons precisely to do that and I certainly intend to continue doing so.”
The BBC reported that a junior Home Office official was suspended from duty 10 days ago over a number of leaks and the matter was referred to police. He was arrested but not charged.
The move follows series of leaks, including:
The revelation 2007 that in November Home Secretary knew the Security Industry Authority had granted licences to 5,000 illegal workers, but decided not to publicise it.
The February 2008 story that an illegal immigrant had been employed as a cleaner inside the House of Commons.
A letter from the home secretary warning that a recession could lead to a rise in crime.
Tory leader David Cameron is furious about what has happened and stands by Mr Green.
Speaking on BBC One’s Question Time, shadow chancellor George Osborne said: “I think it’s absolutely extraordinary that the police have taken that decision.
“It has long been the case in our democracy that MPs have received information from civil servants – I think to hide information from the public is wrong.