Massive changes to Commons constituencies are being announced on Tuesday as part of plans to reduce the number of MPs from 650 to 600 - with Labour expected to be hardest hit.
The initial boundary alteration proposals for England and Wales follow those for Northern Ireland which were disclosed last week. Plans for Scotland are due to be published on October 20.
The number of MPs will be cut from 533 to 501 in England, from 59 to 53 in Scotland, from 40 to 29 in Wales, and from 18 to 17 in Northern Ireland.
The changes will be implemented for the general election due in 2020 - but their full impact will not be known until experts have been able to analyse the commissions' revised proposals in early 2018 and their final proposals in October 2018.
The reductions in England and Wales are expected to have a greater effect in Labour-dominated areas than Conservative ones.
English regions will see the number of their constituencies reduced as follows:
:: Eastern: 57, down one:: East Midlands: 44, down two:: London: 68, down five:: North East: 25, down four:: North West: 68, down seven:: South East: 83, down one overall (two seats are lost but the Isle of Wight goes up from a single seat to two):: South West: 53, down two:: West Midlands: 53, down six:: Yorkshire and the Humber: 50, down four.
Most of the regions where Labour led at the 2010 general election will see bigger reductions. Labour took 32 out of 46 seats in the East Midlands; 45 out of 73 in London; 26 out of 29 in the North East; 51 out of 75 in the North West; and 33 out of 54 in Yorkshire.
Labour won 25 out of the 40 seats in Wales, where more than a quarter of the constituencies are being axed.
Meanwhile three of the four English regions where the Conservatives were ahead in the 2010 poll - Eastern (taking 52 out of 58), South East (79 out of 84) and South West (51 out of 55) - see smaller reductions of one or two. Only the fourth, West Midlands (34 out of 59), sees a bigger cut of six.
This lends support to claims that most of the 43 seats that will go from England and Wales are currently Labour-held.
In Northern Ireland, analysis of the initial proposals indicates the various parties will face tougher competition in many seats, particularly in the three Belfast constituencies replacing the current four.
The reduction in Scottish representation at Westminster by six seats is bound to be felt by the SNP, which took all but three constituencies north of the border at the 2015 general election.
However, reliable estimates of the effects of the proposed changes will not be feasible before 2018. According to the England Boundary Commission, their initial proposals under the previous review changed by 61% by the time they produced their revised proposals.
A media consortium comprising the BBC, ITV News, Sky News and the Press Association has commissioned Professors Colin Rallings and Michael Thrasher of the Elections Centre at Plymouth University to calculate a set of detailed "official" notional results showing how the new parliamentary constituencies might have behaved if they had been in place at the 2015 general election.
These calculations, which can only be completed once the final proposals are known, will form the baseline for reporting 2020 general election results and statistics.
Periodic boundary reviews are deemed necessary to try to minimise variations in the size of constituency electorates.
The previous review of constituency boundaries, which would have brought in a similar reduction in the total number of MPs, was blocked in Parliament by the Liberal Democrats in 2013 following a row with their Conservative coalition government partners. As a result, the 2015 general election was fought on the same boundaries as those in 2010.
The current review uses the electorate figures from December 2015. This has drawn criticism as it excludes significant increases in voter registration seen in the run-up to the EU referendum on June 23.
The commissions insist they have no discretion to use more recent electorate data, although Tom Hartley, Secretary of the England Boundary Commission, has said any evidence of an increase in electorate in a particular area will be taken into account, without going outside the allowed variation.
On the basis of the December 2015 figures, a constituency should have 74,769 voters - but the rules for the review allow a variation of 5% above or below this "quota", meaning constituencies should be no smaller than 71,031 and no larger than 78,507.
Exceptions to this are the Isle of Wight, which is being split into two new constituencies, and other island seats Orkney & Shetland and Na h-Eileanan an Iar (Western Isles). Northern Ireland is allowed some greater variance in principle but the initial proposals for it fall within the desired range.
The boundary commissions for England, Scotland, Wales and Northern Ireland will be carrying out consultation exercises involving public hearings and written representations before producing their final reports and sending them to Parliament in October 2018.