Most constituencies will be fought on new boundaries at the 2010 general election.
These changes will not affect Scotland where new boundaries came into effect in the 2005 election.
The number of seats in the House of Commons will increase from 646 to 650. This means that one party will need 326 seats to hold an absolute majority.
Since the last election, 478 of 533 seats in England, 22 of the 40 in Wales and all 18 constituencies in Northern Ireland will have new boundaries.
The boundary changes are designed to balance the electorate in each seat. They are based on the findings of the Boundary Commission which reviews constituencies every 8 to 12 years to ensure that they represent population patterns.
In order to make fair comparisons with 2005, there are "notional" results, worked out by polling experts, which estimate the votes for each party in each new seat as if it had existed in 2005.