Alan Johnson’s call for a referendum on electoral reform will increase pressure on Gordon Brown to hold a general election before his lease on power runs out.

Senior ministers are already urging the Prime Minister to seize the initiative after the scandal over MPs’ expenses rather than wait for an inevitable defeat next year.

Supporters of Mr Johnson, the Health Secretary, insist that his intervention, in an article published in The Times today, does not sound a call to arms for Labour MPs to depose Mr Brown after the council and European elections next month, in which the party is braced for heavy losses.

Instead Mr Johnson’s allies insist that the suggestion — to hold the referendum on the same day as the poll — could help Mr Brown to catch the public appetite for radical change and outflank David Cameron. The referendum could help to drive a wedge between the Conservatives, who oppose proportional representation, and Liberal Democrats, they claim.

Mr Cameron anticipated the threat yesterday, insisting that “another argument for proportional representation” would be “entirely missing the point”. On BBC One’s The Andrew Marr Show he repeated his call for an early election and said: “What people want is a parliament they can be proud of, but they want more control over their lives at the same time.”

In an attempt to prove his own intent for change, Mr Cameron announced he was reopening the Conservative list of approved candidates to new applications, including from people with no previous involvement in politics, to replace those disgraced in the expenses scandal.

Nick Clegg, the Liberal Democrat leader, said that the next election had to give voters an opportunity to usher in full-scale electoral reform. “We’ve got to use this election to change the rules for good, from top to toe, to make sure the House of Lords is directly elected, to clean up party funding, to make sure the government isn’t voted into power for years with only a fraction of the vote,” he said.

The crisis of confidence has complicated all parties’ electoral calculations, but Mr Brown is still being urged by most advisers to wait for unambiguous evidence that an economic recovery is under way before considering a poll. He is working on a new “national plan” to show how Britain can pay off debt while protecting investment in public services and is likely to reshuffle his ministerial team.

Alistair Darling, the Chancellor, said yesterday that the next election would be decided on a range of issues wider than the economy, including “trust [and] constitutional reform”.

A number of Cabinet ministers have been reported in recent days as supporting a renewed drive to abolish the first-past-the-post electoral system. John Denham, Harriet Harman and Hilary Benn were all said yesterday to be in favour of a debate on the issue.

In breaking cover to call for specific proposals to be put to the voters, Mr Johnson has stolen a march on rival leadership candidates in the Cabinet, including James Purnell and David Miliband. “Its adoption would shift the political focus currently concentrated almost exclusively on a few swing voters in a handful of marginal seats,” Mr Johnson writes.

The argument is a close echo of that put forward by Jon Cruddas, the Dagenham MP who is regarded as a key figure in the next leadership election. “The way to do that is to introduce proportional representation and a system of fair votes to replace the current system, under which a few thousand swing votes have an armlock around the body politic,” Mr Cruddas said yesterday.

The former Home Secretary Charles Clarke called on Mr Brown to end the uncertainty over the general election date. “It would be best now for Gordon Brown to set the election day as the first Thursday in May 2010.”

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be