Independent MP and AM Peter Law was offered a peerage not to stand against Labour in Blaenau Gwent at the last general election, his widow alleges.
Independent MP Peter Law died this week
Labour says her claim is "categorically not true" but Trish Law told BBC Wales that her husband "could not be bought".
The ex-Labour politician, who died this week, was asked not to stand by a senior Labour figure, Mrs Law said.
Mr Law overturned a 19,000 majority to win Blaenau Gwent, Labour's safest Welsh seat at Westminster, last May.
Mr Law, who was 58, died on Tuesday after a year-long battle with a brain tumour diagnosed during last year's general election campaign.
He had been Blaenau Gwent's Labour AM since the first Welsh assembly elections in 1999, but fell out with his party over its decision to select a parliamentary candidate for the south Wales valleys constituency from an all-woman shortlist.
Mr Law was expelled from Labour after his decision to stand for the seat as an Independent.
His widow told BBC Wales' Dragon's Eye programme: "There was pressure put on him. He had quite a number of phone calls from high-ranking politicians not to do it, [they said] he would be silly to do it, there was no way that he would win."
She said her husband told her that he might get a peerage if he did not fight the election as an independent.
Mrs Law said she believed a peerage was a serious proposition, otherwise, she said, "Peter would not have said it to me".
She said: "That's quite a lot of pressure and it was pressure at the time when he was, when he was quite ill as well you know, but Peter couldn't be bought."
Peter Law overturned the largest Labour majority in Wales
When asked, Mrs Law would not name who had offered her husband a peerage.
She added: "I would prefer not to but there were quite a few, there were quite a few politicians who were ringing him on a regular basis, telling him as well that he'd be a fool to stand against the Labour party."
Responding to Mrs Law's comments, a Labour spokesman said: "It's categorically not true. It did not happen."
Another party spokesman said: "Friends of Peter from within the party did plead with him not to stand and that's well known, but there was no offer of a peerage."
But Cheryl Gillan, shadow Welsh secretary, said she would be writing to Prime Minister Tony Blair and Welsh Secretary Peter Hain for "further clarification".
She said: "I find it surprising that the Labour Party is already denying this when they cannot possibly have had a chance to fully investigate the claims in such a short space of time."
Meanwhile, the Labour Party has said it will leave the job of choosing a candidate for the Westminster by-election in Blaenau Gwent caused by Mr Law's death entirely in the hands of the local party.
Another by-election will also take place for Mr Law's seat in the Welsh assembly.
Mr Law's funeral is due to take place at Christ Church in Ebbw Vale next Thursday.