The political map of Britain has been transformed over the past 20 years. What was once mainly coloured blue, with very few red patches outside the big cities and industrial Scotland and Wales, is now largely red in urban and suburban England, with orange patches in the South West and blue largely limited to rural areas.

In 1987, Labour held just over a quarter of the seats in London (23 out of 84), and only one out of 108 in the rest of the South East. Now, Labour has two thirds of the reduced number of MPs in the capital (55 out of 74) and 35 of 117 in the rest of the South East.

In contrast, the Conservatives have been reduced from 58 to 13 MPs in London. They have held on to 73 of their 107 in the rest of the South East.

In 1987, the Tories enjoyed a 7 to 1 advantage over Labour in the South East, including London (165 MPs to 24), but Labour led in 2001 (by 90 to 86, with 15 Liberal Democrats).

The Conservatives have been virtually eliminated in the big industrial conurbations around Birmingham, Manchester, Merseyside, south and west Yorkshire, and Tyne and Wear. They are down from 39 MPs to 5 in these areas, all but one on the outskirts of Birmingham. Liverpool, Manchester, Leeds, Newcastle upon Tyne and Sheffield are all Tory-free zones.

Even outside the big industrial cities, the Tories have been on the retreat, losing more than half their seats in the South West (44 to 20). The Lib Dems now have 16 seats in Devon and Cornwall. Only in the more rural East Anglia region have the Tories limited the drop, from 19 to 14 seats.

The decline of the Tories began in the 1987 election in Scotland and Wales. That year they lost half their seats north of the border and were down to ten, all lost in 1997. They hold just one now. In Wales, Tory representation fell by 14 to 8 in 1987, and has been nil since 1997.

The main reason is simply the reversal in the national position of the two main parties. In 1987, the Conservatives were on 42 per cent of the total UK vote, Labour on 31. In 2001, Labour was on 41 per cent, the Tories 32. But these figures tell only half the story.

Even if the balance of votes between the Tories and Labour was almost exactly mirrored in the two elections, the balance of seats was very different. In 1987, the Conservatives won 376 seats to 229 for Labour, while, in 2001, Labour won 413 against 166. So Labour had 37 more MPs in 2001 than the Tories in 1987 on a slightly smaller share of the total vote, while the Tories in 2001 had 63 fewer MPs on a slightly higher share.

This is because the electoral system has been working strongly in Labour’s favour. Labour now gets many more seats per vote than the Tories, or the Liberal Democrats. This “bias” is the result of the way that votes are cast in particular constituencies. Labour’s vote is concentrated in seats that are smaller than Conservative ones, and it wins more by smaller majorities.

Reviews of parliamentary boundaries every 10 to 12 years have removed many smaller Labour seats in inner cities and have added seats in expanding, more rural areas such as East Anglia and Hampshire. But these have only partially offset the trends helping Labour.

Boundary reviews are historic. The big review introduced at the 1997 election reflected the 1991 census, and the changes likely to come in a 2009 or 2010 election will reflect the 2001 census. In the meantime, further population changes will have occurred. So the average Labour constituency is several thousand smaller than the average Conservative seat.

Tactical voting has further increased Labour’s advantage. This is where voters back their second choice to prevent the candidate of the most disliked party from winning. This has seriously damaged the Tories and helped both Labour and the Lib Dems.

The overall effect of the “bias” in the system has been that, even if the two main parties have the same share of the vote, Labour would have 140 more MPs, at 364 to 224.

According to John Curtice, the election specialist, Labour could be 3.7 percentage points behind the Tories and Tony Blair would retain an overall majority. Conversely, the Tories need an 11-point lead to be sure of an overall majority.

These estimates are heavily dependent on assumptions about past electoral behaviour, in particular a uniform national swing. As Professor Curtice has noted, things could be much less favourable to Labour were there different shifts in target/marginal seats and were tactical voting to unwind.

All this makes it harder to translate opinion polls into projections of the distribution of seats. A further complication is the Lib Dem targeting of both Labour and Tory seats.

Expect the red domination of the map to shrink a little, with blue gains outside the big cities and spots of orange.

Peter Riddell is chief political commentator of The Times

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