Philip Gould, one of the architects of New Labour, has died at 61 of cancer of the oesophagus.
The Labour peer had publicly battled the disease, telling the Guardian in September "you know, this period of death is astonishing."
Gould began working with the Labour party during their slow climb back to power. He advised the party on polling and strategy during their unsuccessful elections in 1987 and 1992, as well as during the victorious 1997, 2001 and 2005.
Tony Blair led tributes to the veteran pollster and strategist, saying he had grown "emotionally and spiritually into this remarkable witness to life's meaning and purpose" while ill.
''Philip was such a huge part of the renaissance of the Labour Party.
''To me he was my guide and mentor, a wise head, a brilliant mind, and a total rock when a storm was raging.
''He became indispensable. He was always a constant advocate for the British people, their hopes and anxieties. So his political contribution was immense.
''I feel very proud and privileged to have known him and to have been his friend.''
Ed Miliband said he was "an exceptional man and his death is an exceptional loss."
"He was Labour to his core, and today, as the Labour Party, we mourn for one of our own.
"He was rightly known as a pathbreaking political strategist. His friends will also remember an extraordinary human being.
"Warm, funny, engaging, deeply emotional and loving to all those he knew. Nowhere was he more extraordinary than in the final years of his life: battling his cancer like a political campaign that could be mastered and won. Then accepting death with courage and bravery.
"His contribution to Labour politics was enormous. His work and commitment helped change the lives of millions of his fellow citizens for the better. By helping Labour to win three elections, he helped rebuild our schools, save our NHS and repair the fabric of Britain. Many who benefit will never know his name but will have better lives in part because of his work.
"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.
"He taught those fortunate enough to know him much about how to live, and in the years of his illness, much about how to die.
"His memory will live on in his wonderful family and all those who had the privilege to call him their friend."
Gould is survived by his wife Gail and two daughters.