Gordon Brown issued a stern warning to Labour’s deputy leadership contenders last night to avoid lurching to the left in their attempt to secure victory.

He told the six hopefuls who appeared alongside him at a party hustings in Leicester that there would be no “retreat to the soft options”.

In a message also intended to counter George Osborne’s claim that David Cameron’s Tories were the natural heirs to Tony Blair’s agenda on the public services, Mr Brown said that Labour would win the next election by continuing to occupy the centre ground.

He said that any government that he led would not back away from reform of the public services. The Chancellor, who has become increasingly impatient with the way that some of the deputy contenders have appeared to pander to the Left in their quest for votes in the party membership and trade union sections of the electoral college, spoke out after a series of “off-message” statements by contenders, four of whom are in the Cabinet.

He is concerned because he is working on the assumption, according to aides, that whoever the party elects as deputy leader will become the deputy prime minister.

He wants candidates to avoid “hostages to fortune” by making comments that are out of line with government policy, so allowing the Tories to make accusations of splits in the leadership.

Some candidates have openly opposed the Government’s decision to renew the Trident nuclear submarine system; others, such as Harriet Harman, have said that they would have voted differently on the Iraq War had they known then what they know now. Peter Hain has attacked City bonuses. Alan Johnson, whom several of Mr Brown’s closest supporters are backing, caused some consternation in the Treasury by suggesting that the tax privileges of private schools could be under threat.

At a union hustings last weekend, three of the contenders irritated Mr Brown by speaking against the closure of Remploy factories, which provide employment for disabled people. One senior minister said yesterday that the deputy contenders were running out of control and that it was time for “Gordon to rein them in”.

Mr Brown, addressing “everyone here and all those in our party”, said that Labour would win the next election, as it had won the last three, by occupying the centre ground of British politics. “There will be no retreat to the soft options, the narrow politics or the failed policies of the past. No retreat from the essential reforms of our public services.” He added: “That is not the way forward for new Labour. That is not the government I will lead.”

Speaking at black and ethnic minority hustings at the Peepul Centre, Mr Brown said that it was wrong that there were only 12 MPs representing such groups and pledged to address the issue as prime minister.

Pressure grew among the candidates to be Mr Brown’s deputy for there to be shortlists made up exclusively of people from black and ethnic minority communities in certain constituencies. Hazel Blears, the party chairman, called on the Labour National Executive Committee “to draw up a code of conduct to move towards all BME [black and minority ethnic] shortlists”.

Peter Hain, the Northern Ireland secretary, also backed the shortlists “where appropriate”.

Minority-only shortlists are illegal under race equality laws, but Labour has unofficially directed its local party to run an all-black women’s shortlist in Ealing Southall, West London. Labour sources said that the party used its legal power to say that the next candidate must be a woman, and then suggested informally to the local party that it should choose a candidate to reflect the seat’s ethnic profile.

Jon Cruddas, the only back-bench candidate, moved into second place in support from constituency parties with 31 behind Hilary Benn on 40.

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