David Cameron insisted police and ministers had "serious questions" to answer after one of his frontbench spokesmen was arrested over Home Office leaks.
The Tory leader hit out after shadow immigration minister Damian Green was held in custody for nine hours on Thursday night as part of a Scotland Yard investigation.
Mr Cameron said: "I think there are some serious questions that have to be answered. If they wanted to talk to Damian Green, why not pick up the telephone and ask to talk to him?"
Mr Cameron raised concerns that at least nine counter-terrorism officers had been deployed to detain Mr Green and search his house and Commons office.
"The police have to answer questions," he said. "Frankly, Government ministers have got questions to answer as well."
Mr Green, 52, MP for Ashford, expressed fury after he was questioned in connection with a series of confidential Home Office papers which have found their way into the public domain recently.
He was released on unconditional bail shortly before midnight without charge, but must return to face further questioning in February.
Speaking to reporters outside the House of Commons, he said: "I was astonished to have spent more than nine hours 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."
Downing Street and Labour have strongly denied that either Prime Minister Gordon Brown or other ministers had prior knowledge that the arrest was taking place.
