Former Liberal Democrat leader Sir Menzies Campbell is to stand down from Parliament at the election scheduled for May 2015.
Sir Menzies - known to all at Westminster as "Ming" - said he had written to Deputy Prime Minister Nick Clegg to inform him of his intention.
He has served as MP for North East Fife since 1987 and was leader of the Lib Dems from 2006-07.
Sir Menzies said: "It's been an enormous privilege to have been an MP for 26 years and to represent such a wonderful constituency as North East Fife. My wife and I have made many friends and have been supported by constituents of all political persuasions and none.
"It is always a regret to begin the process of retiring from the House of Commons but I believe now is the time to start. I have written to Nick Clegg, leader of the Liberal Democrats, and to Harry Wills, chairman of the North East Fife constituency party."
In his letter to Mr Clegg, 72-year-old Sir Menzies said he had "reached the conclusion that now would be the right time to step down and to allow someone else to have the opportunity to serve the people of North East Fife".
He said he would maintain his links with the constituency through positions including his chancellorship of St Andrews University.
He told Mr Clegg, who replaced him as leader in 2007: "Between now and 2015 I will maintain my efforts to be as effective a representative of my constituents and their interests as I can and to the best of my ability support yourself, our party and its interests."
A barrister and former athlete, who held the UK 100 metres record and competed in the 1964 Olympic Games, Sir Menzies entered Parliament as a Liberal MP on his fifth attempt in 1987, and became a Liberal Democrat on the party's creation the following year.
He served on his party's frontbench - most prominently as foreign affairs spokesman - for the majority of his parliamentary career, but his stint as leader following Charles Kennedy's resignation was marked by poor poll ratings and he stepped down after just 19 months. He has most recently served on the House of Commons Foreign Affairs Committee and the Parliamentary Intelligence and Security Committee.
Mr Clegg said it was "a sad day for me, my party and British politics as a whole".
"Sir Menzies has been a towering presence in British politics for the past three decades. He has served this country and our party with unparalleled distinction," said Mr Clegg.
"Most people would be satisfied with just one outstanding career but Menzies Campbell has had three - as an Olympic athlete, a renowned QC, and a leading politician.
"It has been a great honour to succeed Sir Menzies as the leader of the Liberal Democrats. As well as being an outstanding leader, Sir Menzies is a brilliant orator and someone who commands both attention and affection from all sides of the House in Westminster.
"Of his many achievements in politics, I suspect he will be most vividly remembered for his passionate and articulate opposition to the war in Iraq.
"Sir Menzies Campbell has served the people of North East Fife with distinction for an astonishing 26 years. I know he will continue to do so over the next year and a half and in the many other roles he fulfils in the area, such as his Chancellorship of St Andrews University.
"Speaking personally, I have relied for a long time on Ming's enormous wisdom and knowledge and his retirement from Westminster is a sad day for me, my party and British politics as a whole."