Pontypridd MP Kim Howells is leaving government, after 11 years as a minister, in Gordon Brown's reshuffle.
Dr Howells, 61, was a foreign office minister and one of the oldest members of the Labour administration.
He said the prime minister had not explained his decision to him and had told him he was "changing ministers".
Dr Howells takes over from Margaret Beckett as chair of the Security and Intelligence Committee, overseeing the work of MI5 and MI6.
Ms Beckett is returning to government as the new housing minister.
Dr Howells previous jobs have included culture spokesman and higher education minister.
He has regularly made headlines because of his tendency for plain speaking.
He once called the Royal Family "a bit bonkers" and described entries for art's Turner Prize as "cold, mechanical conceptual bullshit".
He embarrassed himself addressing the Commons foreign affairs committee last year when he mistakenly referred to the Israeli prime minister Ariel Sharon as being dead.
Mr Sharon had been in a coma for more than a year.
Speaking on BBC Radio Wales, Dr Howells said he was "not at all" disappointed to be no longer a minister.
"I've got a new job now and one which I know something about," he said.
"I've been working very closely with the security and intelligence services over the last three-and-a-half years and I hope it's put me in good stead," he added.
Destroying records
In 2006 Dr Howells found himself at the centre of public alarm over paedophiles when it emerged he had allowed a registered sex offender to work as a PE teacher.
Dr Howells said he had been told the individual concerned "did not represent an ongoing threat to children" and Downing Street confirmed that he had followed correct procedures.
An MP since 1989, Dr Howells was previously an NUM official during the 1984-85 miners strike and later admitted destroying union records because he was afraid police would raid the office when a taxi driver was killed taking miners to work.
The MP made the confession in a BBC Wales documentary on the strike but he avoided prosecution after going to the police voluntarily after the programme was broadcast.
Writing on his blog, Newport West Labour MP Paul Flynn said: "The sacking of Kim Howells follows a long series of errors and stumbles... his habitat for years has been the last chance saloon".
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