Labour leader Ed Miliband is among the political figures who have paid tribute to former energy minister Malcolm Wicks, who has died aged 65.
The Croydon North MP was praised as a "thoroughly decent, intelligent and compassionate man" whose death left a "huge hole" in the party.
There will now be a by-election in the constituency - a safe Labour seat - which Mr Wicks had served since 1997.
Mr Wicks had been suffering from cancer.
Mr Miliband said: "He was a brilliant energy minister, he was a deep thinker about welfare and pensions, and Labour to his core.
"Malcolm was also a dedicated constituency MP, always putting the needs of his Croydon constituents first. He was a brilliant campaigner.
"I have lost a wise confidant and most importantly a dear friend and the Labour Party has lost one of its sharpest thinkers. Our thoughts go to Malcolm's wife, Margaret, and his family."
Former prime minister Gordon Brown paid tribute, describing Mr Wicks as "a great thinker, writer, campaigner and parliamentarian".
He said: "He was a passionate advocate of social justice, a conscientious constituency MP and a great minister in the Labour government.
"I talked to him during his long period of ill-health and I marvelled at his courage and tenacity in fighting his illness. He will be sorely missed and our thoughts are with his family."
Former deputy prime minister Lord Prescott tweeted: "Our deepest sympathies to Margaret Wicks and her children on the passing of Malcolm. Such sad news".
Communities Secretary Eric Pickles said he was "very sad" to learn of the death of Mr Wicks, who he described as "a man of great wit and humanity".
Lord Adonis, who served in government with Malcolm Wicks, said it was "terribly sad news".
"A highly effective and conscientious MP and minister, will be greatly missed," the former transport secretary added.
Mr Wicks' local Labour Party tweeted: "Malcolm Wicks MP will be missed by so many people. He was so hard working and a true servant of the people of Croydon. RIP."
Before becoming an MP, Mr Wicks worked in the urban deprivation unit of the Home Office, as a lecturer and he was director of the Family Policy Studies Centre.
He held a variety of ministerial briefs under Tony Blair and Gordon Brown - including pensions, science and energy.
The married father and grandfather stepped down from government in 2008 and spent two years as the prime minister's special representative on international energy issues. He was also made a member of the Privy Council.
Mr Wicks won his seat in parliament at the last general election with a majority of 16,483.
The date for a by-election in Croydon North will not be known until Labour's Chief Whip begins the process at Westminster known as "moving the Writ."