The dispute which is likely to begin in the Shadow Cabinet goes to the heart of a problem facing Labour over plans for a Scottish parliament, which critics on all sides have targeted.
Labour is reluctant to admit it may have to cut the number of seats in Scotland because it would slash Labour's representation at Westminster and possibly put at risk a Labour majority if the party won power.
But some of Mr Blair's colleagues believe the over-representation of MPs in Scotland will have to be tackled. "We will have to cut the number of seats. We will just have to make Labour more popular in England," a Shadow Cabinet source said.
There are 72 Scottish MPs, and many have much smaller electorates than MPs south of the border. They include the Liberal Democrat Sir David Steel, whose Tweeddale, Ettrick and Lauderdale seat has only 39,493 voters and Norman Hogg, the Labour MP for Cumbernauld and Kilsyth, with an electorate of 46,489.
Four members of the Shadow Cabinet have Scottish seats: Robin Cook, the Shadow Foreign Secretary; Gordon Brown, the Shadow Chancellor; Donald Dewar, the Shadow Secretary of State for Social Security, and George Robertson, the Shadow Secretary of State for Scotland. Mr Robertson, whose Hamilton constituency has an electorate of more than 60,000, kept to Labour's official line at the weekend that there would be no cuts in representation.
"It is not part of the package to reduce representation of Scotland at Westminster because Westminster will still be dealing with some of the most crucial issues; economic policy, defence and foreign affairs," he said.on Channel 4's a Week in Politics.
Mr Robertson also refused to say which of the two parliaments in which he would prefer to sit.