Alistair Darling acknowledged the slump in Labour’s fortunes yesterday, admitting that the Government needed to “sharpen up” and give a clearer message to voters.

The Chancellor became the most senior figure to call for improvements to Gordon Brown’s administration. He said its slide in popularity was caused by a “whole number of factors”, not the credit crisis alone.

“We will get through this because the economy is fundamentally strong,” said Mr Darling, speaking during a visit to China. “But we have also got to make sure that in other areas we sharpen ourselves up, that we have a clear message of what we are about. All governments and parties go through difficult patches. This is a time when we should remember the purpose of being in government.”

George Osborne, the Shadow Chancellor, claimed that Mr Darling’s remarks were an “unprecedented attack on the Prime Minister by his most senior Cabinet colleague. Even Gordon Brown never criticised Tony Blair in public.”

“What started as anonymous briefings from backbenchers has now burst into the open with a public attack on Gordon Brown from the second most important person in the Government,” Mr Osborne said.

This week the Prime Minister, questioned about his leadership, said that he would not step down at any time soon. The Chancellor dismissed speculation about a leadership challenge. “I see no evidence of it,” he told the financial newswire Bloomberg.

After the Easter recess Mr Darling will return to a Labour rebellion over the abolition of the 10p tax rate, which MPs say is hitting the poor. Backbenchers are likely to seize on his call for Labour to reassert its purpose “to build a fairer society” as they press for measures to compensate those losing out.

The Conservatives are expected to side with Labour rebels over the issue when the Commons votes on April 28. But Mr Osborne came under fire from his own party yesterday after stating that he would “stop tax rises on families”. His spokesman said later that the pledge, in an article in The Sun, did not bind a Conservative administration to reinstating the rate: “He meant that we would do everything we could to overturn its abolition in Parliament in the coming weeks.”

The grassroots website ConservativeHome said that Mr Osborne’s article “verged on the misleading”. “How would he ‘stop the tax rises on families’?” the website asked. “So long as we are following Labour’s spending plans and are committed to pre-fund all tax-cut pledges it is impossible for the party to stop them without raising other taxes,” stated an editorial.

The former frontbencher John Redwood, a leading critic of the Tory commitment to match Labour spending plans, said that Mr Osborne would face questions over how the Conservatives would meet the shortfall.

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