Boundary changes will make it much more difficult for Labour to win an overall majority in the next general election, according to a new analysis.
But the Conservatives will still need a swing of about 10 per cent to win power outright, making a hung parliament more likely than for many years, research published in the House Magazine suggests.
Of 13 new seats created by the Boundary Commission, 10 would have been won by the Conservatives in 2005, compared to two for the Liberal Democrats and just one for Labour.
Meanwhile, six of the nine seats to be abolished at the next election are Labour-held, compared to just two Tory and one Lib Dem.
The analysis by researchers Lewis Baston and Simon Henig suggests that several Labour seats in the south will be transformed into wafer-thin marginals by boundary changes.
A swing of just over one per cent will now cost Labour its overall majority, compared to 1.8 per cent with boundaries unchanged.
The changes reduce the swing needed by David Cameron to secure an overall majority from 11 per cent under the old boundaries to nine or 10 per cent with the constituencies which will be used in the next general election, expected in 2009 or 2010.
The boundary changes have been driven by population shifts which are seeing voters move gradually into the suburbs from the inner cities.
Labour seats which will become more vulnerable to the Tories include Enfield North, Selby, Sittingbourne and Sheppey,
Staffordshire Moorlands and Wansdyke, the researchers said.
Based on ward-by-ward analysis of voting patterns in 2005, seats which are likely to switch from Labour to Conservative because of the changes also include Finchley and Golders Green, Medway, Thanet South
and Wirral West.
A further nine constituencies which will change from reasonably safe Labour seats to vulnerable marginals include Basildon, Conwy, Crosby, Ealing Acton and Shepherds Bush, Hampstead and Highgate, Milton Keynes South West, Regents Park and Kensington North and Sunderland South.
New seats which could be expected to go Tory include Mid-Derbyshire, Devon Central, Kenilworth and Southam, Northamptonshire South, Witham, Wyre and Preston North, Filton and Bradley Stoke, Meon Valley, Norfolk Mid and Broadland and York Outer.
But Labour can expect to have a better chance of seizing Lancaster and Wyre, Northampton South, Rugby and Kenilworth, Croydon Central and Kettering from the Tories under the new arrangements.
Constituencies being abolished include Normanton, held by Labour's economic secretary to the Treasury Ed Balls, Liberal Democrat education spokeswoman Sarah Teather's Brent East and Vale of York,
held by Conservative work and pensions spokeswoman Anne McIntosh.
Henig said: "The boundary changes, which are more or less complete, will make a difference of about 20 seats.
"There is still going to be a big electoral bias against the Conservatives, but the size of swing that Labour can afford has halved."