Hazel Blears stoked the growing tensions in the Labour deputy leadership race by telling her rivals that pandering to the Left would play into the hands of David Cameron.

She told one rival, the back-bencher Jon Cruddas, that his cautious support for a 50p tax rate for high earners was electoral suicide.

Another contender, Alan Johnson, was dealt a blow when his old union blocked a move to support him. The conference of the Communication Workers Union, of which Mr Johnson used to be the general secretary, censured a decision by the executive to endorse his candidacy.

Ms Blears, the Labour chairman and most Blairite of the six candidates, hit out in an interview with The Times after a week in which some of her rivals have variously opposed the war in Iraq; called for an amnesty for illegal immigrants; proposed an end to tax exemptions for private schools; opposed Trident; and attacked City bonuses.

Ms Blears, who seems not to mind being called the “on-message candidate”, said: “Elections are won and lost on the centre ground of politics. We vacate that ground at our peril.”

Referring to a speech last week by George Osborne, the Shadow Chancellor, in which he claimed that the Tories were best placed to carry forward the Blairite reforms, she said: “We have seen Osborne and Cameron trying to clamber on to our ground . . . It is a mistake to think that the party wants you to play to the Left. While the heartbeat of the party may be to the left of the public, its head recognises that you have to take the marginal seats to win the next general election.”

Ms Blears’s concerns surfaced as Mr Cruddas, the only nonminister in the race, said that he was not hostile to the introduction of a 50p top rate of tax for those earning more than £100,000 to fund Labour’s spending commitments.

Opposition to the backing for Mr Johnson came from supporters of Peter Hain, the Northern Ireland Secretary, and Mr Cruddas.

The executive of the union will now meet at its annual conference in Bournemouth to decide whether to recommend a choice of candidates not including Mr Johnson or not to recommend any candidate at all.

Hazel Blears

Ministerial career Rapidly upwards. Junior Health Minister, Home Office Minister, Labour Party chairman

Political outlook Blairite moderniser

As deputy leader would speak up for Labour members in Cabinet; act as campaigner-in-chief; deliver Labour’s manifesto

Famous for being a child extra in A Taste of Honey; riding classic motorcycles

Working-class credentials Impeccable. Father was factory fitter, brother is bus driver. She turned down assisted place at private school. First in family to go on to higher education

Trade union links Fair. Was Nalgo representative when working as local authority solicitor, now member of Usdaw

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