Political journalist and commentator Anthony Howard has died after a short illness, his family says.
The 76-year-old former editor of the New Statesman worked for newspapers including The Times and Guardian, and appeared on TV as a Westminster pundit.
Former Prime Minister Gordon Brown said Howard's "eloquence, wit and beliefs straddled 50 years of our history".
His death had robbed the country of "one of its best informed and talented commentators", David Cameron said.
The prime minister said he had been "saddened" to hear the news.
Made a CBE in 1997, Howard edited the diaries of former Labour cabinet minister Richard Crossman, and wrote biographies of Mr Crossman and Conservative chancellor and home secretary "Rab" Butler.
Mr Cameron said: "His work on the Crossman diaries was ground-breaking and his biography of Rab was superb. He will be greatly missed."
Former deputy PM Lord Heseltine said Howard "had an encyclopaedic knowledge of contemporary British politics".
Lord Heseltine, who had known Howard since they attended Oxford University together in the 1950s, said: "It is a terrible shock. Tony was one of my oldest friends.
"He was a very kind and warm-hearted person with a deep, deep compassion."
Labour leader Ed Miliband said: "Anthony Howard was not just a distinguished political commentator, but someone who conveyed the excitement and importance of the events on which he was commenting."
Labour MP John McDonnell described Howard as "vastly knowledgeable" and "gifted".
And on Twitter, Daniel Finkelstein of the Times said he had been "a brilliant editor and commentator too".
Howard was known for roles on the BBC's Panorama and Newsnight. He also hosted Face the Press on Channel 4 and The Editors on Sky News, and appeared on BBC radio.
His views were much in demand during this year's and previous general elections.