The Fifth Periodic Review of Westminster constituencies was undertaken between 2000 and 2007 by the four boundary commissions for England, Scotland, Wales and Northern Ireland for the UK Parliament. The changes for England, Wales and Northern Ireland took effect at the 2010 United Kingdom general election; that for Scotland took effect at the 2005 election. All of the recommendations were approved.

The boundary commissions were required at the time by the Parliamentary Constituencies Act 1986 to review constituencies in their part of the United Kingdom every eight to twelve years. The Commissions' recommendations from the review were based on the numbers of electors on the electoral register and ward boundaries at the start of the review in 2000.

In Scotland, the recommendations were submitted in November 2004, and approved in February 2005.^[1] In Wales, the recommendations were submitted on 31 January 2005, and approved on 11 April 2006.^[2] In England, the recommendations were submitted on 31 October 2006, and approved on 13 June 2007.^[3] In Northern Ireland, the recommendations were submitted on 14 September 2007 and approved on 11 June 2008.^[4]

These were the first major changes to UK Parliamentary constituencies since 1997.

As set out in the approval dates above, the Scottish changes were effective from the 2005 general election. The post-2010 shape of those for the rest of the UK can be viewed alongside those for Scotland of 2005 at United Kingdom Parliament constituencies.

A side effect of reviews is the number of seats won by each party may change (even if all voters repeated their votes at later elections).

The total number of Scottish seats dropped from 72 to 59, due to changes made by the Scotland Act 1998, applying the English electoral quota to Scotland. This had reduced total seats back down^{[lower-alpha 1]} from 659 to 646, pending the review of the rest of the UK.

In Wales, the total seats remained 40, but entailed radical redrawing in Clwyd and in Gwynedd. Aberconwy, Arfon and Dwyfor Meirionnydd replaced Conwy, Caernarfon and Meirionnydd Nant Conwy respectively. 18 seats were unchanged with only minor changes elsewhere. Per 2006 statistics, Welsh seats had on average 16,000 fewer electors than those of England.

In Northern Ireland, the Boundary Commission recommended that the number of seats remained at 18, with minor changes to 12 of these, primarily in the east of the province.

In England, the seats recommended seldom straddle the largest council areas, i.e. counties (metropolitan or otherwise); however they may cross boundaries of unitary authorities. York was consolidated to have two seats, without electoral wards from North Yorkshire. The unitary authorities of Berkshire are represented in Westminster by many cross-authority seats.

The Boundary Commission for England created one extra seat each to represent ten non-metropolitan counties: Cornwall, Derbyshire, Devon, Essex, Hampshire, Lancashire, Norfolk, Northamptonshire, Warwickshire and Wiltshire; and one covering the area of the former county of Avon. This was partly offset by the abolition of one seat in each of the six metropolitan counties of Greater Manchester, Merseyside, Tyne and Wear, South Yorkshire, West Midlands and West Yorkshire, together with a net loss of one seat in Greater London.

London was reviewed borough-by-borough – some were "paired". This means they see one or more straddling seats. This solution ensures that the sizes of the electorates are not too disparate, in other words, quite fairly apportioned to reflect the adult resident population. Two boroughs lost a seat and one area of central London gained a constituency.

The approved recommendations for the three countries saw a net increase of 4 seats from 646 to 650 (for the 2010 general election).

The reduction in the number of seats from 72 to 59, combined with the 1996 reorganisation of local government under the Local Government etc (Scotland) Act 1994, resulted in a significant reconfiguration of constituencies in Scotland. Only three seats remained unchanged: East Renfrewshire (formerly named Eastwood), and quota-immune Orkney and Shetland, and Na h-Eileanan an Iar (formerly named Western Isles).^[5] A further 22 seats were retained with increased electorates of varying amounts, with 47 abolished constituencies being offset by 34 new ones.

The table below lists all Scottish seats before and after the redistribution, linking new and abolished seats where applicable, as described above.

• Sixth Periodic Review of Westminster constituencies
• List of United Kingdom Parliament constituencies