A new website will pair up Labour and Lib Dem voters around the country in a vote swapping system based on the so-called Nader trading agreements between Democrats and Greens in the American presidential race.
Labour supporters in Tory-Lib Dem marginals will pledge to vote Lib Dem, provided Lib Dem tactical voters in close Tory-Labour seats agree to vote Labour.
The system, which matches voters by email, is intended to allow supporters to avoid casting a "wasted vote", while keeping their conscience clear that their party's national vote share will not be affected by their tactical manoeuvres.
The site, supporting the centre left but not affiliated to any party - and judged by the Electoral Reform Society to be "just on the right side of the law" - is aimed at building on the kind of informal tactical voting that is widely believed to have lost the Conservatives seats at the 1997 election.
Founders of the vote trading website www.tacticalvoter.net and other new sites encouraging tactical voting within constituencies argue that the power of the internet and email can make tactical voting more effective than ever.
Jason Buckley, the businessman who created the site and is a supporter of voting reform, said: "Tactical voting was just crying out for the technology to make it happen, and the internet can do that by linking people up."
People would still want to avoid wasting their votes in single party strongholds, he predicted, suggesting that the site could potentially affect results in some 15 marginals, such as Torbay and Winchester, which are narrowly held by Lib Dems and which the Tories aim to recapture.
Tacticalvoter.net is funded by the Joseph Rowntree Reform Trust and by the voter education campaign New Politics Network.
Another site, stophague.com, encourages tactical voting to retain the Lib Dem and Labour MPs Ed Davey and Roger Casale in, respectively, Kingston and Surbiton, and Wimbledon, both formerly Tory.
