Liberal Democrat peer Lord Roper has died aged 80 after what the party said was "a long battle with illness".
The peer, who retired from the House of Lords in May 2015, was the party's chief whip there from 2001 to 2005.
He was MP for Farnworth, Lancashire from 1970 to 1983, representing Labour and then the breakaway Social Democratic Party (SDP).
Dick Newby, his successor as the party's chief whip in the Lords, said "he will be deeply missed".
"John was, throughout his life, a great servant to social democracy and liberal politics in Britain.
"He was a passionate believer in the European project. He always fought for the idea that Britain's best interest was working together with our friends across Europe to build a better future for us all.
"John served as chief whip in both the Commons and the Lords and proved to be a natural in the role. He was a great mentor of mine, a fine European and a widely respected chief whip," Mr Newby said.
Lord Roper held a senior staff role at the Royal Institute of International Affairs, Chatham House, and was the first director of the Institute for Security Studies of Western European Union in Paris.
He was Labour's spokesman on defence from 1979-81 and SDP chief whip from 1981 to 1983.
From 2008 to 2012 he was principal deputy chairman of Committees of the House of Lords and chairman of the European Union Committee.