However, the recommendation for change, expected tomorrow, would be a significant turning point for PR campaigners. They believe that well before the next election, Labour must present voters with the prospect of a fairer, more representative system for future parliaments.
The working party's final report is also likely to contain a section on the need to promise wide-ranging public discussion before any changes are brought in - a constitutional convention, for example - and for the Parliamentary Bill implementing them to contain provision for a referendum.
Lord Plant, the chairman, has taken firm steps to stop the 'no change' faction of three to five members led by Margaret Beckett, the deputy party leader, manipulating final decisions to block the views of the majority.
Members were still arguing yesterday about the voting system the committee should use before putting its recommendations about voting systems before next month's ruling National Executive Committee. But at tomorrow's meeting, Lord Plant, professor of politics at Southampton University, is expected to insist that members vote on a 'grid' of combinations of proportional representation systems for the Commons, a revised second chamber to replace the House of Lords, the European Parliament and a Scottish Parliament.
That is calculated to cut down opportunities for 'tactical' voting that could fudge the result. Some members said they would also press for an initial vote for or against retaining the present system for electing MPs, to send a clear message to the NEC and the autumn party conference.
In a letter to all members, Professor Plant has emphasised that making clear recommendations formed part of the working party's terms of reference. Failure to do so could be construed as a damaging failure.
About half of the pro-change members of the 18-strong committee, including Professor Plant, favour a mixed-member PR system. This is a variation of the additional-member system that would see 500 MPs elected by first-past-the-post plus 150 'best losers' selected from lists to reflect proportionately parties' shares of the vote in regions. Its opponents claim that it would give too many seats to 'roving' MPs, particularly Liberal Democrats, without constituency connections.
Roughly similar numbers back a supplementary vote system. This would allow voters to exercise first and second preferences, but all MPs would be linked to constituencies. Opponents say it would involve an unacceptably high degree of tactical voting and would do little to tackle under-representation of Labour MPs in the South.
The balance appeared to be tipping towards the supplementary vote yesterday.
One member said the vehemence of Mrs Beckett's opposition to changing the Commons system had turned former supporters against her. But the predictions yesterday were that a slender majority of the pro- change group would plump for the supplementary vote, on the basis that the party as a whole would not support a system that weakened constituency links.