Shadow immigration minister Damian Green was arrested and questioned by Metropolitan police for nine hours yesterday.
His home and office in Kent were searched along with an address in West London. Green's office at the House of Commons was also searched. It is believed the investigation is linked to the leaking of embarrassing information from the Home Office - a Home Office official was arrested last week in connection with leaked documents.
David Davis told the BBC the case was reminiscent of Robert Mugabe's Zimbabwe.
A spokesman for the Met told The Register: "A 52-year old man was arrested in Kent yesterday he has been bailed to return to a central London police station in February."
A spokesman for the Tory Party said: "As Shadow Immigration Minister Mr Green has, on a number of occasions, legitimately revealed information which the Home Office chose not to make public. Disclosure of this information was manifestly in the public interest."
"Mr Green denies any wrongdoing and stands by his actions."
It is highly unusual for police to arrest serving MPs - an informal interview is the more normal route. Home Office officials insist the investigation was started after they complained to the Met and that no ministers were involved.
The leaks are believed to include: revealing that Jacqui Smith knew the Security Industry Authority had given licenses to 5,000 illegal immigrants, that an illegal immigrant was working as a cleaner in the House of Commons and a letter from Jacqui Smith warning that a recession would likely lead to an increase in crime.
Despite the denials it seems highly unlikely that the Met police would make such an arrest without informing ministers.
All the more so, after it emerged that Mayor of London Boris Johnson was told in advance of the operation and had expressed "trenchant" concerns.
A statement from Johnson's office said: "The mayor finds it hard to believe that on the day when terrorists have gone on the rampage in India that anti-terror police in Britain have apparently targeted an elected representative of Parliament for no greater crime than allegedly receiving leaked documents."®