Britain's longest-serving MP, Sir Peter Tapsell, is to stand down at the next general election.
The Conservative will tell his local association tonight that he will not fight the 2015 campaign.
Sir Peter, 84, was first elected to the Commons 55 years ago aged 29 and has contested 15 parliamentary elections in five constituencies.
As a result of his lengthy parliamentary service, Sir Peter is Father of the House, a title bestowed on the MP who has served in the Commons for the longest time without a break.
A former stockbroker and merchant banker, he was a persistent critic of Margaret Thatcher's economic policy and was named as the Tory MP to second Michael Heseltine's nomination for the leadership of the Conservative Party in 1990.
Sir Peter was personal assistant to Sir Anthony Eden during the 1955 general election campaign. He went on to fight, and lose, a by-election at Wednesbury, Staffordshire, in 1957, but won West Nottingham from Labour in 1959, only to lose it in 1964.
He was a front-bench Opposition spokesman on Treasury and Economic Affairs and on Foreign Affairs in the 1970s.
Sir Peter's decision to stand down will open up a plum vacancy in a safe Tory seat. In the 2010 election he secured a 13,871 majority.
Speculation will inevitably focus on Boris Johnson, particularly after David Cameron said today he wants the mayor of London back in the Commons at the next election.
Mr Johnson has always insisted that he will see out his term in the capital, due to end in 2016, but the Prime Minister suggested that he "can stay on as mayor and come back to the House".
The mayor's official spokesman would not be drawn on the speculation but said: "The mayor is getting on with running London - delivering the housing, the jobs and the growth needed in a rapidly-expanding city."
Sir Peter was elected MP for Horncastle in 1966 and has served the constituency under its numerous boundary and name changes - most recently Louth and Horncastle - ever since.
After him, there are four MPs - Sir Gerald Kaufman, Kenneth Clarke, Michael Meacher and Dennis Skinner - who share the longest unbroken service, having first been elected at the 1970 election.
Commons Speaker John Bercow said: "Peter has been a magnificent credit to his constituency, to his party and to his country.
"He respected the House and, in turn, the House respected him.
"His politics have been grounded in Keynesian economics, social concern and passion for the British Parliament.
"He is a fine orator with a great wit, whom I have come to know and greatly admire over the last 17 years."
Prime Minister David Cameron said: "Sir Peter will be greatly missed as father of the House by his colleagues in his own party and right across Parliament.
"He has brought colour and life to the House of Commons, and is a fount of knowledge and anecdote drawn from a Parliamentary career spanning six decades.
"His love of this country, its history and its democracy has always shone through - not least in the penetrating questions he continues to put to ministers and prime ministers week in, week out.
"He has become almost a parliamentary institution in his own right.
"I wish him all the very best for the future."