Lib Dem, Labour and SDP political veteran Shirley Williams had died aged 90.
Baroness Williams was in UK politics for more than half a century, serving in the Labour government of Harold Wilson before becoming one of the 'Gang of Four' rebels who broke away to form the Social Democratic Party in 1981.
The SDP later merged to form the Lib Dems, for whom she was a peer in the House of Lords for more than two decades from 1993, retiring in 2016.
She died peacefully in the early hours today, the Lib Dems announced.
Party leader 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.
"Political life will be poorer without her intellect, her wisdom and her generosity. Shirley had a limitless empathy only too rare in politics today; she connected with people, cared about their lives and saw politics as a crucial tool to change lives for the better."
Lib Dem leader in the Lords Dick Newby said: “Shirley Williams was a charismatic, committed and fearless politician who believed passionately in a fairer society and in Britain’s European destiny.
"She was an inspiration and mentor to many younger politicians and maintained her campaigning zeal to the end.
"She was central to the formation of the SDP and the Liberal Democrats, and will be greatly missed by many in her extended political family.”