Alan Johnson launches a daring attempt to lead Labour’s renewal today, urging Gordon Brown to offer voters a referendum on electoral reform.

Mr Johnson, regarded by the bookmakers as a strong favourite to succeed Mr Brown, calls on the Prime Minister to seize the political initiative after the expenses scandal.

“The current public mood of anger and disquiet . . . demands a response,” the Health Secretary writes in The Times today. In an article setting out his modernising credentials, he adds: “We need to overhaul the engine, not just clean the upholstery.”

Although allies insisted yesterday that his intervention was not intended to challenge Mr Brown, it will inevitably strengthen suspicions that a contest over the Prime Minister’s replacement is under way.

In breaking cover to offer Mr Brown advice on the next election, Mr Johnson has stolen a march on possible candidates such as David Miliband and James Purnell. Ed Miliband, the Climate Change Secretary, made a weekend call for reform of the political system, including more power for local government and a shake-up of Prime Minister’s Questions.

In his article Mr Johnson dares Mr Brown to take up an issue abandoned by Tony Blair when, as Prime Minister, he shelved an official report calling for the introduction of proportional representation (PR).

Voters should be given a choice between the “elegant” option of Alternative Vote Plus and the present first-past-the-post system in a referendum held alongside the next general election, he says. “This is a genuinely radical alternative that only Labour in government can facilitate.”

Downing Street, already digging in for a leadership crisis after the council and European elections on June 4, insisted that it was unconcerned about the Health Secretary’s call. “Alan Johnson is a well-known and longstanding supporter of proportional representation,” one of Mr Brown’s aides said. A Labour Party source said that PR had little support among MPs or activists.

One friend of Mr Johnson denied any ulterior motive in the article. “He would hardly launch a leadership bid by backing a position that has historically had little support in the Parliamentary Labour Party,” he said.

The intervention will be seen by others, however, as a sign of Mr Johnson’s growing confidence, as well as a riposte to those who have questioned the 59-year-old minister’s ambition and modernising zeal.

David Cameron rejected attempts to “dust down” arguments over electoral reform as “entirely missing the point”. Instead he announced that he was reopening the list of approved Conservative Party candidates to replace MPs quitting in disgrace over their expenses. He gave only qualified support to Julie Kirkbride after accepting the resignation of her husband, Andrew MacKay.

Times Recommends

• Why Witherspoon is highlighting domestic violence
• No, MP does not stand for Mostly Plastered
• Don’t bottle out of your champagne moment