Ex-Liberal Democrats leader Paddy Ashdown has died aged 77 after a short illness, a party spokesperson has said.
The former MP for Yeovil, who "made an immeasurable contribution to furthering the cause of liberalism", passed away on Saturday evening, a statement said.
Lib Dem leader Sir Vince Cable said Lord Ashdown had "made a real mark" on Parliament and it was "a hugely sad day".
The longest serving leader of the Lib Dems in its history, Lord Ashdown led the party between 1988 and 1999, when it became a growing force in UK politics.
Former Lib Dem leader and ex-deputy PM Sir Nick Clegg said Lord Ashdown was "the most heartfelt person I have known".
"He was the reason I became a liberal. And he became a lifelong mentor, friend and guide."
Former Lib Dem leader Tim Farron said: "Paddy Ashdown was a hero to me, he saved and revived the Liberal Democrats at our lowest ebb, and then led us to our best result for 70 years.
"Only three months ago he was absolutely buzzing with energy and ideas at our party conference".
The party's leader in the House of Lords, Dick Newby, said Lord Ashdown was "a natural leader" who "kept the Liberal Democrats alive in our early years".
Ex-Conservative prime minister Sir John Major hailed his former opponent as "a man of duty, passion, and devotion to the country he loved - right up to the very end".
He added: "In government, Paddy Ashdown was my opponent. In life, he was a much-valued friend."
Labour leader Jeremy Corbyn said Lord Ashdown would be "greatly missed".
Lord Ashdown was an influential figure within the party and a strong supporter of Nick Clegg's controversial decision to take the party into coalition with the Conservatives in 2010.
Prior to entering Parliament in 1983, he served as a Royal Marine and in the intelligence services.
After standing down as an MP in 2001, he served for four years as the United Nations' high representative in Bosnia-Herzegovina.