England will lose 31 MPs, Wales 10, Scotland seven and Northern Ireland two under changes to reduce the size of the House of Commons from 650 to 600, it has been announced.

The figures were calculated during talks between the independent Boundary Commissions for the four nations of the United Kingdom, which are beginning an 18-month project to redraw constituency maps across the country.

Officials have said that the majority of constituencies will be changed - creating a nerve-wracking situation for MPs who hope to fight the next election but cannot be sure whether their own seats will survive.

The Parliamentary Voting System and Constituencies Act, which was passed into law after lengthy wrangling between Commons and Lords last month, sets out complicated formulas for ensuring that almost all seats are roughly the same size.

No constituencies will cross national borders, and the Boundary Commission for England has said it will aim to ensure that none cross regional boundaries.

But critics say that the Act's requirement for all but a few seats to contain between 72,810 and 80,473 voters will inevitably mean crossing county, council and ward boundaries and overriding historic loyalties.

Provisional maps of the proposed new seats will be published in the autumn for consultation, before the commissions produce their final recommendations, which must be completed by autumn 2013 in time for use in the general election expected in 2015.

England's representation at Westminster will drop from 533 to 502 MPs, Wales's from 40 to 30, Scotland's from 59 to 52 and Northern Ireland's from 18 to 16.

Within England, the North West will lose seven MPs and London and the West Midlands five each. Four MPs will go in Yorkshire & the Humber, three in the North East and two in each of the South East, East of England, East Midlands and South West. The Isle of Wight's single seat - currently the largest in the Commons - will be split into two, as one of a handful of island areas exempted from the population rules.

Other exceptions have been made for Na h-Eileanan an Iar (the Western Isles), and the Orkneys and Shetlands, which will remain as island constituencies with smaller populations than those in the rest of the UK. And a rule limiting the size of constituencies to 12,000 square kilometres will probably preserve Charles Kennedy's Ross, Skye and Lochaber, which has an electorate of just 50,000 despite being the UK's largest seat.