It seems the sticking point in the Lords deadlock over the Parliamentary Voting and Constituencies Bill is how much bigger or smaller than average MPs' constituencies should be.

The bill is supposed to address one of the Conservatives' key grievances over the current electoral system - that Labour seats are often much smaller than Conservative seats - and so Labour are over-represented in the Commons. So they want the constituency map redrawn so that the electorate of most constituencies (there are a few exceptions, Orkney and Shetland, the Western Isles and the Isle of Wight) are kept to within 5% of a standard size - what's known in the trade as the electoral quota.

Labour thinks that is too inflexible and will lead to illogical and even ridiculous constituency boundaries. They have argued for some way of relaxing the 5% rule in exceptional circumstances. And following the precedent by which the other two major conflicts in the bill have been resolved (over some form of public hearing for controversial boundary changes and over whether there should be some independent assessment of how many MPs there should be in the House of Commons) they have asked the Crossbench or independent peers to come up with a proposal.

So far the legal eagle Lord Pannick has come up with two draft amendments setting out what would be considered exceptional circumstances in which greater variation would be permissible. And the government has not accepted either.

So it may well be that the issue has to be fought out in the chamber next week. But if that battle is time consuming, or if Labour peers once again dig their heels in, the bill may not be passed in time to allow the proposed referendum on changing the voting system to be held in May.

Or, alternatively, the government may judge that it has enough support to break with the Lords' long tradition and impose a timetable on the remainder of the bill to force it through by the Monday after next. The whole idea of limiting their debates in this way horrifies almost all peers - they normally revel in their ability to debate at length, but with self-discipline, so that everything they want to examine is examined - but legislation goes through on time. If that tradition were to be scrapped, it would be a huge constitutional change. Life would go on, the earth would not shudder on its axis: but while the immediate effects would be intangible, in the end they would be quite profound. The Lords would process legislation in a different way, and therefore produce different, and possibly worse law. And in the long term, that could matter rather a lot.

UPDATE: Lord Pannick has confirmed that he has put down his amendment - but without government endorsement. Another key vote now looms next week.

