Tony Blair has paid tribute to Mo Mowlam, saying she was "one of the most remarkable and colourful personalities" in the history of British politics.
Politicians across the spectrum have also praised her achievements.
The ex-Northern Ireland minister died on Friday at the age of 55, at Pilgrim House Hospice in Canterbury, Kent.
She was moved there from King's College Hospital, London, a week ago, having failed to regain consciousness after falling and hitting her head at home.
Ms Mowlam is understood to have suffered balance problems as a result of treatment for a brain tumour she had originally been diagnosed with in 1997.
Food and water were withdrawn earlier this week, in accordance with her wishes, to allow a natural death. She had asked not to be resuscitated.
The prime minister said Ms Mowlam had been "great company, utterly irreverent, full of life and fun".
"Yet behind that extraordinary front presented to the world was one of the shrewdest political minds I ever encountered.
"She was a natural politician, could read a situation and analyse and assess it as fast as anyone."
Ms Mowlam's successor as Northern Ireland secretary, Peter Mandelson, said her achievement had been "to see in the Good Friday Agreement in Northern Ireland, having broken barriers through the strength of her strong personality.
"This legacy is a permanent tribute to her."
US ex-senator George Mitchell, who chaired the Good Friday Agreement talks, said: "Mo Mowlam made a major contribution to the peace process in Northern Ireland at a crucial time when little progress was being made."
Current Northern Ireland Secretary Peter Hain echoed those sentiments.
"I firmly believe that Northern Ireland is a better place for Mo Mowlam having been there," he said.
Former US president Bill Clinton said he and his wife, Hillary, were saddened by Ms Mowlam's death.
Mr Clinton said: "Mo was an integral part of building a peace process in Northern Ireland that has endured for over a decade.
"Her persistence, toughness and good humour were legendary. All of us who worked to support peace in Northern Ireland owe her our gratitude."
The Labour MP for Redcar between 1987 and 2001, Ms Mowlam helped organise Mr Blair's leadership bid in 1994 and became his cabinet "enforcer" in 1999 after turning down the job of health secretary.
But after becoming increasingly disaffected with his premiership, she became a vocal opponent of the war in Iraq and stood down as an MP in 2001.
Former cabinet minister Clare Short described Ms Mowlam's death as "a very sad day indeed".
"Mo's been ill for some time, but it's still such a shock to lose her so young. It feels unfair and wrong. She was so full of life and sparkle," she said.
Former Labour leader Neil Kinnock said Ms Mowlam would want to be remembered as a "hell of a woman" and described her as "serious, smart, fun and a fighter".
Deputy Prime Minister John Prescott said Ms Mowlam "combined a fine intellect with a straightforward, no nonsense approach that spoke directly to people".
And Ken Livingstone, the mayor of London, said: "To lose both Robin Cook and now Mo Mowlam in a matter of weeks must make this one of the saddest of months for the Labour movement in a very long time."
Conservative leader Michael Howard said Ms Mowlam would be "remembered fondly".
And Liberal Democrat leader Charles Kennedy praised her "great courage and deep humanity both in her public life in politics and privately".
SNP Leader Alex Salmond said Ms Mowlam had the "rare ability to give politics a human face".
Ms Mowlam also drew praise from outside of Westminster.
Pop star Elton John said she was "one of the most charismatic politicians of our time".
And artist Rolf Harris, who recently met Ms Mowlam for a BBC programme, said: "I can't believe she's no longer with us. She was such a livewire, vibrant and full of fun."
Ms Mowlam had for 18 months been the agony aunt for men's magazine Zoo, and Deputy Editor Ben Knowles said she had been "great" at the job.
"I think the fact that it was slightly perverse, was going to shock people and surprise people attracted her to it," he said.
Ms Mowlam had been married since 1995 to Labour-supporting merchant
banker Jon Norton, who already had two children.
Her funeral will be a private family occasion, but a memorial service will be
held in a few months' time.