Roy Hattersley, the former deputy leader, and Gerald Kaufman, the former shadow Foreign Secretary, launched a campaign for "first-past-the-post" voting to be retained for Parliamentary and local elections.
Mr Kaufman said: "You have the bizarre possibility of a Labour Party, out of opposition after 18 years, asking its newly elected, victorious, triumphant, and exultant backbenchers to abolish their constituencies, which they have fought very hard to win.
"There are some members of the Shadow Cabinet who have voiced their support for proportional representation. I would be interested to know which of them would be willing to give up his seat to Alan Beith, Paddy Ashdown, or Menzies Campbell."
The clear message to Mr Blair was that many incoming Labour MPs would not vote for any proportional representation Bill under a Labour Government.
Mr Kaufman said that if Labour had an overall majority of one after the General Election, introducing proportional representation for the Commons would require 34 Labour MPs to vote for the abolition of their own seats. Every Labour MP added to the majority would be expected to abolish his own seat, Mr Kaufman said.
Some key members of the First Past the Post Group believe a Blair government could be defeated, if it attempted to pass the legislation, through a combination of Labour rebellion and Tory opposition.
Mr Blair has stopped short of embracing electoral reform for the Commons, but Labour is committed to offering a referendum on PR. The group is mounting its campaign now, through trades unions and Labour constituencies, to reverse the pressure for PR.
Mr Hattersley said that in any system of PR there was domination by smaller parties.
"I joined the Labour Party to change the nature of society. That objective cannot be achieved by a coalition government and coalition governments are the inevitable outcome of PR."