Baroness Williams of Crosby – better known as Shirley Williams – was one of the disenchanted ex-Labour Cabinet ministers who became the Gang of Four founders of the breakaway and short-lived Social Democratic Party.

She was a busy, breathless, tousle-haired intellectual who acquired an unwanted reputation for missing trains or going to the wrong venue for meetings. That was how she became affectionately known as Shilly Shally Shirley.

Once Lady Astor told her: “You will never get anywhere in politics with hair like that.”

And although in her early political life she surprisingly regarded herself as left-of-centre in Labour terms, she came to be reviled by the party’s left who denounced her as a traitor to the movement after her defection to the SDP.

This was when, along with scores of other Labour Party members, she became appalled at the leftward lurch of the movement, and quit to help form what was dreamed of as the all-conquering party of the centre ground.

Throughout her political career, both in the Labour Party and subsequently the SDP and then the Liberal Democrats, Williams was a passionate pro-European and a fierce opponent of those who took a contrary view.

She is largely remembered for her period as Education Secretary in the years before Margaret Thatcher swept to power. She is blamed to this day as the architect of the controversial comprehensive system.

At one time, there was serious talk of Williams becoming Britain’s first woman Prime Minister, but it was not to be. She herself outwardly showed no ambition in this direction and she was anyway viewed, in political terms, more as a perpetual lieutenant rather than a general.

Shirley Vivien Teresa Brittain Williams was born on July 27 1930 into a privileged household in Chelsea, with two living-in servants. Her mother was Vera Brittain, a prominent feminist and author of Testament of Youth.

Her father, Sir George Catlin, a teacher of political science and unsuccessful Labour candidate, used to wheel Shirley to Labour meetings in a pram.

She was educated at the Summit School, Minnesota, USA, where she was evacuated during the war, St Paul’s Girls School, London, and Somerville College, Oxford, where she met Bernard Williams, then a philosophy student and later a don.

Before going to University, however, Williams took a number of jobs to gain experience being in turn a land girl, a waitress and a chambermaid.

While working as a waitress in Northumberland, she organised a strike and won higher wages for her fellow staff. That was at the age of only 17.

At Oxford she became the first woman chairman of the University Labour Club. She also revealed herself as a capable actress, touring America with the Oxford University Players in 1950, playing Cordelia in King Lear.

She also won a scholarship for postgraduate study at Columbia University, New York.

The couple married in 1955 when she was 25 and they had a daughter Becky.

After a brief and troubled flirtation with journalism on the Daily Mirror, she threw herself into politics.

Her first foray was as unsuccessful Labour candidate at a by-election at Harwich in February 1954 and again at the 1955 general election, but she attracted attention by substantially improving Labour’s share of the poll.

She also fought Southampton Test in 1959 again without success.

Williams eventually entered Parliament as MP for Hitchin in 1964 and held junior office during most of Harold Wilson’s first administration.

Her big opportunity came early. In 1966, as junior minister at the Ministry of Labour, she had to take over, for several weeks, from Ray Gunter, the Secretary of State, who had to rest through illness.

Her success during this period in the so-called “bed of nails” in dealing with the aftermath of the seamen’s strike and handling the bitterly fought Selective Employment Act, led people to predict that she would one day become the first woman Prime Minister.

She said at the time that she felt complacent, almost invincible, both on the political and domestic fronts, but admitted later she had made the mistake of not working at her marriage.

In 1970, her world crashed. Her mother died and her husband announced he had fallen in love with someone else. He divorced her in 1974: a bitter blow in every respect, not least because she was a practising Catholic.

“It seemed so happy that I believed the fact that we did not meet much would not make a difference. I now know you should never, ever, take relationships for granted,” she said afterwards.

Meanwhile, in political terms, she was climbing through the ranks of the Wilson and Callaghan Cabinets of 1974 to 1979 first as Prices and Consumer Protection Secretary, then as Paymaster General and finally Education Secretary.

She had served on Labour’s ruling National Executive Committee between 1970 and 1981.

But after Margaret Thatcher’s arrival at 10 Downing Street, and her own defeat in 1979, Williams began to have increasing doubts and disillusionment about the way Labour was lurching to the left.

Finally, along with William Rodgers, David Owen and Roy Jenkins, she helped to form the SDP – a new party which claimed “unstoppable momentum” in its public support.

But it was not to be. The party, of which she was president for a while, eventually collapsed amid recriminations and was moulded into the Liberal Party which, through a series of name changes, finally became the Liberal Democrats.

Before that she had in 1981 fought and won a by-election at Crosby to become the SDP’s first elected MP. She lost the seat in 1983 and failed again when she fought Cambridge at the 1987 general election.

Meanwhile she had met Professor Richard Neustadt, a distinguished US academic and former adviser to Presidents Kennedy and Carter. They married in 1987, although she was inhibited by her deep Catholic beliefs.

The following year she took up the post of Professor of Elective Politics in the John F Kennedy school of Government at Harvard University, so they could more easily be together.

She became a life peeress in 1993 and sat in the House of Lords as a Liberal Democrat and continued to remain deeply immersed in the political scene.

In the autumn of 2004, Baroness Williams retired as the Liberal Democrat leader in the House of Lords.

She warned against a formal coalition with the Tories in 2010, but remained loyal to leader Nick Clegg after his tie-up with David Cameron, defending the Lib Dem leader in the face of efforts to oust him.

She retired from the House of Lords in 2016, bowing out of parliamentary politics with a heartfelt plea for Britain to stay in the EU, five months ahead of the referendum that delivered Brexit.