Michael Ancram, the Tories' new deputy chairman, told a news conference that the current system provided stability and accountability and urged the government to stick with it.

The prime minister, Tony Blair, has set up an independent commission to look into alternative voting systems. The commission, under the chairmanship of Lord Jenkins, is expected to report in December 1998.

Mr Ancram thinks the commission will not give adequate consideration to the current system and that there should be more debate: "It is a rigged commission that will provide a rigged result. We are not opposed to the setting up of the commission but we want to see an even, level playing field."

The Tories will keep the pressure up in the House of Commons as they use an Opposition day motion to debate the issue.

Mr Ancram and the newly appointed shadow constitutional affairs spokesman, Liam Fox, will use Tuesday's debate, entitled "The dangers of changing Britain's successful and fair electoral system", to unveil new figures based on information taken from the Commons library to back their claims that the current British system is more "proportional" than the alternatives.

Dr Liam Fox: "It works better than any other system so No to PR"

Mr Ancram said: "What matters in a democracy is whether people get the government they voted for. Under the British electoral system, your vote counts.

"This is the heart of our case against PR. It would give small parties a pivotal role in forming governments, allowing them to exercise a degree of power quite disproportionate to their share of the vote. PR often means that the largest party is excluded from office."

However, Tim Rathbone, Tory MP for Lewes between 1974 and 1997, has condemned the debate: "The Parliamentary motion on electoral systems ... must be one of the most thoughtless and complacent of political actions by any party for a very long time.

"It forgets the lesson of history. In the February 1974 election Conservatives were denied government because Labour won four more seats even though Conservatives had 230,000 more votes than Labour.

Andrew Puddephat, director of 'Make Votes Count', which is launching a campaign in support of PR on Tuesday, told BBC Radio 4's Today programme that people felt politicians were "too remote, they bicker too much, they don't co-operate, they are unaccountable and out of touch."

Puddephat: "People feel their vote doesn't count..."

Mr Puddephat said that his campaign's survey suggested that people also felt it was unfair that their votes "didn't count".

"We found seven out of 10 people want to change the system," he added.

He went on to say that voters were not greatly concerned by fears that minority parties could win power, because if people wanted majority government, it just meant half had to vote for one party.