Former cabinet minister and Liberal Democrat peer Shirley Williams – one of the "Gang of Four" who defected from Labour in the early 1980s – has died aged 90.
Baroness Williams of Crosby – whose long parliamentary career began when she was first elected an MP in 1964 – was said to have "died peacefully" in the early hours of Monday, according to a Lib Dem spokeswoman.
Tributes have poured in for "the undisputed godmother of the Liberal Democrats", who spent more than half a century in Westminster politics.
Lib Dem leader Sir Ed Davey said: "This is heartbreaking for me and for our whole Liberal Democrat family.
"Shirley has been an inspiration to millions, a Liberal lion and a true trailblazer.
"I feel privileged to have known her, listened to her and worked with her. Like so many others, I will miss her terribly."
And former Labour prime minister Tony Blair described Baroness Williams as "one of the greatest social democrats of the last century" and "an immense figure of progressive politics through the decades".
"For many of us in the Labour Party and even after she left it, she remained a source of inspiration and someone to look up to and admire – warm, generous, humane, and uplifting," he added.
Boris Johnson called Baroness Williams a "kind and thoughtful member of the once radical left" who "will be much missed".
He said they often disagreed but she always had "the gift of sounding so completely reasonable".
Baroness Williams first entered parliament as Labour MP for Hitchin in 1964, after which she went on to serve in Harold Wilson’s governments in the late 1960s and mid-1970s.
She was a member of Mr Wilson’s cabinet and, when he was succeeded as Labour leader and prime minister by James Callaghan, Baroness Williams served as education secretary until 1979.
But, at that year’s general election, she lost her seat in the House of Commons.
Less than two years later, Baroness Williams and three other former Labour cabinet ministers defected from the party to found the Social Democratic Party (SDP) in 1981.
The "Gang of Four", as they became known, were unhappy with Labour’s move to the political left under Michael Foot – who was a supporter of nuclear disarmament and wanted Britain to withdraw from the European Economic Community.
They also bemoaned how "a handful of trade union leaders can now dictate the choice of a future prime minister".
At a by-election in Crosby later in 1981, Baroness Williams became the first MP to be elected as an SDP candidate.
Her success came at a time when both Labour and the Conservative government were divided and with the SDP being supported by the Liberal Party.
That electoral pact – known as the SDP-Liberal Alliance – lasted through the 1983 and 1987 general elections.
The former saw the Alliance win more than a quarter of the national vote but only have 23 MPs elected, while Baroness Williams lost her Crosby seat.
The 1987 election saw the Alliance’s vote share fall slightly as Labour produced a much stronger showing.
Following this, the two parties agreed on a formal merger to become the Social and Liberal Democrats and then, later, the Liberal Democrats.
In 1988, Baroness Williams moved to the US to teach at Harvard University before accepting a peerage in 1993.
She served as Lib Dem leader in the House of Lords from 2001 to 2004.
From their Twitter account on Monday, the Lib Dem group in the House of Lords posted: "We are desperately sad at the passing of Shirley Williams – the undisputed godmother of the Liberal Democrats, without whom the party probably wouldn’t exist."
Labour’s shadow justice secretary David Lammy described how he was "lucky to be taught by her at Harvard and to forge a friendship across the political divide in politics".
"Kind, eclectic with the sharpest of minds she was a female pioneer and a giant of the political centre ground," he added.