Tory proposals for a US senate-style second chamber to replace the House of Lords would give Labour 144 senators - just seven short of an overall majority and 63 more than the Conservatives, finds an analysis for The House Magazine.Iain Duncan Smith will this week flesh out his plan for "a 300-member assembly, to be called the senate".

"A clear majority, 240 of its 300 members, would be elected by first past the post in 80 county-wide constituencies," wrote the Conservative leader last week - spelling an "historic shift" for the Tories on the question of Lords reform.

"The remaining 60 members would be nominated by an independent appointments commission answerable to parliament, not to Downing Street," he said.

The Conservatives, who count 221 hereditary and life peers among their parliamentary number, will propose transitional arrangements, phased in over three general elections, during which members of the House of Lords would continue to sit.

Labour criticism of the proposals will focus on the equal representation given to counties of varying sizes, which as Duncan Smith puts it, "is reminiscent of the medieval and pre-1832 House of Commons in which each county elected two 'knights of the shire'".

An analysis for House Magazine by the PA news elections editor, Chris Mead, based on the results of last year's general election, suggests that an IDS-style senatorial vote would have given Labour 144 seats, the Tories 81, the Liberal Democrats nine, Ulster Unionists three and Sinn Fein three.