The independent MP and AM for Blaenau Gwent, Peter Law, has died at 58, after suffering from cancer.
Mr Law left the Labour Party in protest at its policy of all-women candidates' shortlists and overturned its 19,000 majority at the 2005 general election.
He had been suffering from a brain tumour, first diagnosed during the election campaign.
Mr Law, a former chairman of Gwent Healthcare NHS Trust, leaves a wife Trish, two sons and three daughters.
He died peacefully at home early on Tuesday morning. His funeral will be held at 1000 BST on 4 May in Christ Church, Ebbw Vale.
There will now be by-elections in the constituency - which up until 2005 had been Labour's safest parliamentary seat in Wales - for both the assembly and Westminster.
Abergavenny-born Mr Law started as a grocer and became a local councillor in Blaenau Gwent at the age of 22, rising to the position of council leader.
He was elected as an assembly member at the first assembly election in 1999 and served in Alun Michael's cabinet as environment and local government secretary, before being sacked by successor Rhodri Morgan in a reshuffle in 2000.
But an internal Labour row was brewing in Blaenau Gwent over an all-woman shortlist being imposed in the parliamentary constituency, where Michael Foot and Aneurin Bevan had preceded retiring MP Llew Smith.
Labour argued it needed to increase the number of women MPs from Wales, but it created deep divisions.
Most local party members boycotted meetings to choose from the five women candidates at the end of 2003, with only 100 out of 700 voting.
Mr Law joined protests as the Wales Labour Party took over the selection process.
A party member since he was a teenager, he eventually decided he would stand against union official Maggie Jones as an independent in the general election.
But he dramatically withdrew early in the campaign after discovering he had a brain tumour.
His wife had persuaded him to visit a doctor, on the eve of his declaration to stand, after complaining of regular headaches.
After recovering from six hours of surgery and with two weeks to polling day, he eventually decided to press ahead.
He won a spectacular victory against Labour by a majority of 9,121 votes.
Mr Law's inevitable expulsion by Labour meant the party lost its working majority of one in the Welsh assembly. There was still division in his constituency, where 20 of his supporters were also expelled by Labour.
PETER LAW MP, AM 1948-2006
Born 1 April 1948, Abergavenny
Married to Trish, two grown-up sons, three daughters
Blaenau Gwent councillor 1974-1999
Blaenau Gwent AM 1999-2006
Blaenau Gwent MP 2005-2006
Local govt and environment minister 1999-2000
Member of the Welsh Language Board 1990-1995
Former chair Gwent Healthcare Trust and JP
One of only two independent MPs in the Commons, Mr Law was named as the Welsh politician of the year at the Welsh Political Awards last December.
He said he had been "very humbled" by the honour after "a rollercoaster year".
Mr Law's death was reported to the House of Commons by Speaker Michael Martin.
At the start of business, Mr Martin told the house: "I'm sure members on all sides will join me in mourning the loss of a colleague and extending our sympathy to his family and friends."
His words were echoed by shadow foreign secretary William Hague and by Michael Moore for the Liberal Democrats.
Flags at the assembly offices in Cardiff Bay were flying at half-mast on Tuesday in tribute to Mr Law.