When MPs return to Westminster from their extended summer break today, one subject will preoccupy their thoughts. It will not be the downfall of Muammar Gaddafi's regime, the plans to reform the NHS or the changes to abortion law, all of which are on this week's Commons agenda.
Instead their minds are turning to moves to redraw the UK's electoral map, as plans are published to scrap 50 of the 650 parliamentary constituencies and alter the boundaries of the vast majority of the rest. Scores of party colleagues will suddenly find themselves pitched into battles to contest merged seats, while hundreds of MPs will be fighting such radically revised constituencies that previously comfortable majorities cannot be guaranteed.
The Boundary Commission's initial proposals for England, Scotland and Northern Ireland, which are due to be published next week, will be a matter of political life and death for Westminster's politicians – many of whom only arrived at the Commons last year.
The planned overhaul has left MPs of all parties anxious and apprehensive. That mood could turn to fractiousness among backbenchers – particularly those with little chance of preferment – who see their survival in peril.
Labour, which accuses the Coalition of gerrymandering, will oppose the changes. Both the Tories and Liberal Democrats accept they face rebellion from disgruntled MPs – in the form both of Commons votes and parliamentary wrecking tactics.
Yesterday the Conservative strategist Robert Hayward produced a list of only 61 constituencies that he confidently forecast would remain unchanged. They include the seats held by the cabinet ministers William Hague and Liam Fox, as well as the constituency of the Labour leader, Ed Miliband. Mr Hayward said that the shake-up could present the Coalition with the toughest test of its unity.
The reduction in seats was agreed by the Liberal Democrats in return for David Cameron's promise to hold last May's referendum. Now Nick Clegg's party risks losing a disproportionate number of seats as a result of the boundary review without the compensation of electoral reform. A senior party source said: "We will have rebellions. It will be a long and difficult process."
The Government says that Britain has the largest elected chamber in western Europe and that cutting the size of the Commons will save the taxpayer £12m a year. Conservatives also argue that the current system has an inbuilt bias towards Labour MPs, whose constituencies typically have smaller electorates than their Tory counterparts.
The Boundary Commission will set out proposals for parliamentary seats of between about 73,000 and 80,500 voters, with four constituencies – the two new Isle of Wight seats, Orkney and Shetland and the Western Isles – spared from the quota. Wales, whose proposals will be produced next year, will be particularly hard hit, losing 10 of its 40 seats. Because of the rigid new formula for calculating constituency size, the Commission will for the first time have to cross county and council boundaries.
That has angered MPs of all parties, with one Labour former minister complaining: "MPs will often have to get to know about two very different places with their own local issues."
The hot seats
* The most high-profile battle could come between two of the Liberal Democrats' best-known names – Charles Kennedy, the former party leader, and Danny Alexander, the Chief Secretary to the Treasury.
Their vast under-populated constituencies in the Scottish Highlands are likely to be merged. Neither is anywhere near retirement age, so they could be forced to go head-to-head unless fellow Lib Dem John Thurso can be persuaded to step down in Caithness.
* Two Shadow Cabinet colleagues, Ed Balls and Hilary Benn, face the prospect of slugging it out to represent the seat that emerges from their corner of West Yorkshire.
* Two Labour rising stars, Stephen Twigg and Luciana Berger, could see their Merseyside seats merged.
* A number of prominent Tory MPs, including Graham Brady, the chairman of the 1922 backbench committee; Grant Shapps, the Housing minister; and Hugh Robertson, the Sports minister, could find their constituencies vanish or heavily redrawn.
* Ed Miliband and David Cameron are not expected to be affected by the overhaul. However, the Chancellor George Osborne could face sweeping changes to his Cheshire constituency.
* The boundaries of Nick Clegg's constituency will probably be substantially changed, but the majority of his Sheffield seat is likely to remain unchanged.