With Labour's Plant committee in session, and under pressure from the party's electoral reformers to produce proposals for change, Mr Maclennan said STV remained his party's 'firm preference'.
But he acknowledged that 'there is no perfect system' and 'we would have to look with serious interest at whatever is proposed to see whether it meets the criteria'.
At present, there is no possibility of Labour backing STV - the system used in Irish elections and for the European Parliament in Northern Ireland - for the House of Commons. Indeed, the Plant committee has already effectively ruled it out. Its considerations today centre on a version of the alternative member system, where about 500 MPs would be elected on first past the post, but with their numbers 'topped up' by the best losers in each region to give a more proportional result.
Mr Maclennan said it was not clear that this would provide the 'truly proportional' system which the Liberal Democrats sought, but added 'we are not blinkered and are willing to listen to the arguments'. Party sources accept that if Labour does abandon its historic commitment to first past the post, any alternative system has to pass through the existing House of Commons. Mr Rooker's proposal is designed with that in mind, producing the minimum change needed to get greater proportionality by leaving 500 constituency- based MPs topped up by the 150 regional members.
Lord Plant is expected today to push his committee to a decision - either agreeing the outline of a consensus report recommending change, or if that cannot be achieved, agreeing to majority and minority reports that would at least keep the debate within Labour's ranks alive.
Senior Labour MPs - and it is thought Lord Plant himself - are anxious that the report should not be reduced, like its trade union study, simply to proposing options for change.Reuse content