Conservative immigration spokesman Damian Green has accused police of arresting him "for doing my job" after he spent nine hours in custody answering questions about leaks of politically sensitive information from the Home Office.
The Ashford MP "emphatically" denied any wrongdoing and vowed to continue bringing to light embarrassing information which ministers tried to keep secret.
Senior Conservatives, including leader David Cameron, are known to be furious about Mr Green's arrest at his home in Kent by the Metropolitan Police officers, who were believed to have included members of the counter-terrorism team.
Mayor of London Boris Johnson revealed that he was informed of police plans in advance and voiced "grave" concerns to acting Met Commissioner Sir Paul Stephenson, warning him that he did not regard it as "common sense policing".
Mr Cameron is expected to raise questions about the way the case was handled in an early-morning address before the media.
Senior Tory sources were describing the operation as "Stalinesque" and suggesting that police must have received authorisation from the very top of the Government.
But Downing Street and Labour issued strong denials that either Prime Minister Gordon Brown or other ministers had prior knowledge.
And the Metropolitan Police Service released a statement insisting that "the decision to make today's arrest was taken solely by the MPS without any ministerial knowledge or approval".
Mr Green, 52, was questioned at Belgravia police station in central London over a series of confidential Home Office papers which found their way into the public domain over recent months. He was released on unconditional bail shortly before midnight without charge.
Speaking to reporters outside the House of Commons, he 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."
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