Former Labour Welsh assembly member Peter Law has confirmed he is to stand as an independent in the party's safest seat in Wales at the general election.
Peter Law is standing in Blaenau Gwent, Labour's safest Welsh seat
Mr Law had originally dropped plans to fight his old party in his Blaenau Gwent seat because of a brain tumour and underwent surgery some 10 days ago.
But then Mr Law, 57, said on Friday he felt well and was considering standing.
On Monday, the day before nominations will close, he announced that he definitely would be a candidate.
The announcement poses two immediate problems for Labour. It will have to defend a seat regarded as rock solid, and it has now also lost its working majority in the assembly.
Mr Law also confirmed on Friday that he had left Labour, which had held 30 of the 60 assembly seats. Although he will still be an AM, he is now an independent.
Even if he had not said he was leaving Labour, under its rules he is considered to have expelled himself by announcing that he would stand against an official candidate.
The Blaenau Gwent AM initially threatened to stand 18 months ago if Labour used an all-women shortlist to choose a successor to the retiring MP, Llew Smith.
Labour chose Maggie Jones from an all-women shortlist
The selection of Maggie Jones, a union official with Unison, led to the resignations of most of the local party executive. The party argues that there are too few women in parliament, and special measures are needed to redress the balance.
Mr Law told BBC Wales: "I made a contract with the people of Blaenau Gwent 18 months ago... when they imposed this all-women shortlist by positive discrimination, and they refused to listen to us. Today I'm announcing I will stand.
"I was encouraged by many prominent people who have said they will work for me.
"I'm very grateful to them because they are very high-principled people and they can see the point that we have been used and abused."
He said it was a chance for Blaenau Gwent votes to send a message to Tony Blair that they were not going to be "taken for granted and used".
Maggie Jones told BBC Wales: "I very much regret that Peter's taken this decision.
"But the choice is about the next government - do people want to go back to the Tories or go forward with Labour?
"I believe people will in large numbers vote Labour in this constituency."
Plaid Cymru's candidate in the constituency, John Price, said: "I'm looking forward to challenging both Peter Law and the New Labour candidate in this election.
"The people of Blaenau Gwent have a clear choice between two parties on 5 May."
Mike German, the Liberal Democrat's assembly leader, told BBC Wales: "I think this throws the whole thing open.
"I know there is a great disaffection about the Labour Party here."
Nick Bourne, Conservative leader in the Welsh assembly, said: "Peter Law's decision to stand is symbolic of the Labour Party's willingness to ride roughshod over local opinion."
On 3 April, the day before Mr Law originally was set to announce his candidacy, he discovered he had a brain tumour.
He dropped his plans to contest the seat, and had surgery later that week.
Labour is defending a 19,313 majority in the seat, which has been held by the NHS' founding father, Aneurin Bevan, and former Labour leader Michael Foot.