Liberal Democrat MP Jeremy Browne has announced he will stand down at the next general election.
The MP for Taunton Deane, who served as a Foreign Office and Home Office minister, said he would explore "a world beyond beyond politics full of opportunities".
In a letter to his local party, posted on Twitter, he said that "in national politics, my race is run".
Mr Browne, who was first elected in 2005, had a 3,993 majority in 2010.
After the coalition government was formed, he served as a minister of state in the Foreign Office before becoming minister for crime prevention in 2012.
He was fired from that post in the Lib Dems' reshuffle in October last year.
After leaving government, he was at odds with the party leadership when he called for the top rate of tax to be lowered to 40p, a policy Lib Dem Chief Secretary to the Treasury Danny Alexander had said would happen "over my dead body".
In his resignation letter, Mr Browne said: "For the avoidance of doubt, I will not be joining another political party and I have no intention to serve in any other capacity in politics."
Ruth Bradley, BBC Somerset politics reporter
In 2010 Jeremy Browne was the first person to win consecutive general elections in Taunton Deane since 1992, a constituency which had flip-flopped between the Conservatives and Liberal Democrats for 18 years.
He was so surprised one of his first comments to me at the declaration was along the lines of: "Well I'm the only MP with a superfluous 'e' on the end of my name and my constituency's!"
In his first ministerial role he gave me a tour of the Foreign Office including his office which he told me had two ornate wooden doors, historically in case two Maharajah of equal status arrived at the same time.
Previously rumoured as a challenger for Lib Dem leadership there's a particularly telling line in his resignation letter: "I have come to the conclusion that in national politics my race is run."
A spokesman for Nick Clegg said the Lib Dems "wish him all the best for the future", adding: "The deputy prime minister regrets that he has taken the decision to leave politics as Jeremy has always had strongly held views which he expressed with great skill and conviction.
"Jeremy has been a tireless constituency MP to the people of Taunton and served in two important ministerial roles in the early part of this government."
Gill Slattery, chairman of the Taunton Deane Liberal Democrats, said: "Obviously we are sad that he's chosen to move on to a different career next year. However we are a lively and buoyant constituency party and expect to have a candidate selected by Christmas."