Labour peer, strategist and corporate adviser Philip Gould has died aged 61 after a battle against cancer.
One of the architects of New Labour, he was seen as playing a crucial role in helping former prime minister Tony Blair to three election victories.
Paying tribute, Mr Blair said: "He was my guide and mentor, a wise head, a brilliant mind."
In a recent BBC interview, Lord Gould said his message was to "have faith and try and change the world".
Speaking on the BBC's Andrew Marr Show, he spoke movingly of what he called the intensity of being in a "different place", knowing that he would not recover from a recurrence of cancer of the oesophagus.
Lord Gould, a former advertising executive, was recruited to the party by Peter Mandelson - then Labour's communications director.
The pair met at a dinner party in 1985 and he was invited to help reshape the party's presentation.
As well as advising then party leader Neil Kinnock and becoming one of Mr Blair's most influential advisers, he also told Gordon Brown before he became prime minister that he needed to be "a powerful, muscular modernisation politician".
In his tribute, Mr Blair said his former adviser had been a "huge part of the renaissance of the Labour Party".
He said Lord Gould "was always a constant advocate for the British people, their hopes and anxieties", and a man whose "political contribution was immense".
The former prime minister added: "But then as his illness gripped him, he became something more. In facing death, he grew emotionally and spiritually into this remarkable witness to life's meaning and purpose.
"No-one who saw him in those last months was unchanged by him. And the bond between him and his wonderful family was a joy to see. I feel very proud and privileged to have known him and to have been his friend."
Gordon Brown also paid tribute to Lord Gould, saying he was "proud to count him as a friend".
"Philip Gould was a brilliant political strategist, a renowned pollster, a superb adviser - but he was much more, a man of strong principles and high ideals who communicated to all around him an infectious belief in a Labour party which rose to the aspirations of the British people," he said.
"Even when seriously ill he continued to fight for the ideals he believed in."
'Driving a revolution'
Lord Gould championed the use of focus groups, made up of ordinary voters, to track public opinion and reaction to particular policies.
Former Labour communications chief Alastair Campbell said he was "fundamental" to Tony Blair's team.
"Philip had phenomenally strong beliefs. He brought those to the table and he worked his guts off for them," he told the BBC.
But Mr Campbell said that the label he acquired, "Tony's Blair's favourite pollster", did not "even begin to describe the scale of his contribution", describing him as "fundamental" to the whole New Labour mission.
Former business secretary, and another New Labour architect, Lord Mandelson said: "Philip was as brave in his illness as he was in his politics, always doing things differently.
"For Labour, he was instrumental in driving a revolution that transformed not just our party but British politics as a whole.
"When he became ill with cancer, instead of retreating into himself, he took it on and, in the process, took everyone along with him."
Labour leader Ed Miliband said Lord Gould's death was "an exceptional loss".
"In an age when people are cynical about politics, he was someone who was in it for the best of reasons: because of his deep-rooted concern for the people of Britain and his wish to make Labour their voice.
"I know from our conversations over the last year he would have made a big contribution to Labour in the years ahead because he had an extraordinary ability to understand changing times and how politics could and should respond to that call."
Lord Gould, who died at the Royal Marsden Hospital in London on Sunday, had undergone treatment for cancer of the oesophagus, but the illness later recurred.
He is survived by wife Gail Rebuck and two daughters, Georgia and Grace.
Details of a funeral service, to be held in London, will be announced later, the family said.