After forty years in politics, Shirley Williams steps out of the limelight

Baroness Williams of Crosby, one of the most respected politicians of her generation, is to step back from front-line politics after 40 years and retire as leader of the Liberal Democrats in the House of Lords.

Baroness Williams of Crosby, one of the most respected politicians of her generation, is to step back from front-line politics after 40 years and retire as leader of the Liberal Democrats in the House of Lords.

A founding member of the SDP who, as Education Secretary, was a staunch proponent of comprehensive education, she is planning to announce her resignation in September. But the Liberal Democrat peer, 72, is not expected to completely abandon public life.

A firm believer in public service, she once said that her parents "instilled into me that if you'd had advantages - birth, education, family - you had to repay them."

Lady Williams's greatest legacy to British politics is her split from Labour as a member of the "gang of four" with Roy Jenkins, Bill Rodgers and David Owen, which led to the creation of the SDP. Although she recently said she had no regrets about leaving Labour, she declared she still shared its "commitment to social justice".

Affectionately known as "Shirl the girl", she was involved in politics from her earliest years, campaigning for the Labour Party even before she could vote. The daughter of Professor Sir George Catlin and Vera Brittain, whose First World War memoir Testament of Youth imbued in a generation the horror of war, she came into contact as a child with the leading literary figures of the time.

As a child she was evacuated to the United States and, unlike many politicians of her generation, has always been a proponent of fostering transatlantic understanding, alongside European integration. After attending the girl's public school St Paul's, she went toSomerville College, Oxford.

Shirley Vivien Teresa Brittain-Catlin first entered Parliament in 1964, as Labour MP for Hitchin, and later served as a minister in the governments of Harold Wilson and Jim Callaghan. A famously feisty politician, her favourite song is the "Battle Hymn of the Republic", which was sung on her wedding day to her late husband Professor Richard Neustadt, the American political scientist.

As Education Secretary, she argued for a comprehensive system that believed in equality of outcome. But this policy prompted a backlash from the electorate and she lost her seat at the 1979 general election.

Her break with Labour helped to shape British politics today, but damaged her former party and arguably helped keep it out of office for 18 years. In her letter of resignation, she said: "The party I loved and worked for over so many years no longer exists."

The famous Limehouse declaration established the SDP and was followed by the famous photograph of a smiling Gang of Four. In 1981, Shirley Williams returned to the Commons as SDP member for Crosby, only 30 months after losing her seat as a Labour MP.

She returned to Parliament in 1993 as a Liberal Democrat peer and has since taken a prominent role on the Liberal Democrat benches, serving as deputy leader of the party in the Lords from 1999, before becoming its leader in 2001.

Lady Williams is expected to step down in November and her decision to retire from the post has been an open secret in the House of Lords for months. Although the resignation will not be formally announced until before the party conference in September, there is already talk about who will take over her role in the upper house.

Lady Williams, who was in Moscow yesterday, is expected to be succeeded by the popular Liberal Democrat peer Lord McNally, currently her deputy.

The front runners to become the deputy leaders are Lord Dholakia, a Charles Kennedy loyalist who, as president of the Liberal Democrats, has worked hard to promote women and ethnic minority candidates in the party; and Lord Wallace, a respected specialist in international relations.

She will be able to spend more time walking or taking long cycle rides on her old Raleigh, which she says she does when she is "annoyed". But, although she is unlikely to drift away from the public eye and will undoubtedly continue to write and maintain her media profile, her ebullient presence in the leader's seat is expected to be missed.

Lady Williams has given the party an added gravitas in the second chamber and has become the party's elder statesman, going out to bat for the Liberal Democrats on BBC2's Newsnight and BBC1's Question Time in her distinctive bright yellow jacket.

Since her husband died at the end of October, Lady Williams has continued her relentless schedule. She recently said she continues to work so hard because "he would have wanted it".

As Liberal Democrat leader in the Lords, she has been a critic of the Government's war in Iraq. She has also be critical of the treatment of captives in Guantanamo Bay and the danger of eroding civil rights in Britain in the name of battling against terror.

Lady Williams is a familiar figure in the Lords and, although she has been at the forefront of British politics for decades, she remains approachable and ungrand.

Behind the scenes in the Liberal Democrats she has been a wise and loyal counsellor to Charles Kennedy, frequently sending him missives full of advice.

But, a seasoned political operator, she has not flinched from publicly criticising the party if she believes it is moving in the wrong direction

The most prominent female politician in the party, she has long been an outspoken critic of her own party's failure to do more to promote women in politics. At a party conference in 2001, when the party voted against affirmative action for women candidates, she warned that the Liberal Democrats were in danger of writing the "second-longest suicide note in history", a reference to Michael Foot's doomed 1983 Labour manifesto.

Her commitment to women's causes runs in Lady Williams's family. Her grandmother, Edith Catlin, was an early suffragette while her mother was a vocal supporter of votes for women.

Recently Lady Williams has criticised the "adolescent" style of politics in Britain, which she said remained "schoolboyish" and responsible for turning off many women from participating in politics and even voting.

Asked recently if she regarded her political career as a success, Lady Williams replied, with characteristic humility: "Part success, part failure, I never became leader or deputy leader of either party I belonged to. But I think what I mean by success is having led a deeply satisfying life in terms of feeling what you have done with it has been worthwhile."