Some of Britain's leading economists are warning the chancellor, George Osborne, that the economy is too fragile to withstand his drastic spending cuts and that he must draw up a plan B.

Experts, including two former Whitehall advisers and two signatories of last year's high-profile letter backing the Tories' cuts, have told the Observer that they have profound concerns about the direction of Treasury policy.

Since the chancellor laid out his plans to balance the books by the end of the parliament in his "emergency budget" a year ago, the outlook has deteriorated markedly. Growth has gone flat over the past six months and a slew of dismal data has raised fears that the UK could be sliding towards a double-dip recession, as the US recovery wanes and the Greek debt crisis rattles the eurozone.

Jonathan Portes, the director of the National Institute of Economic and Social Research, who until February was chief economist at the Cabinet Office, advising the prime minister, said: "You do not gain credibility by sticking to a strategy that isn't working."

He said that the recent slowdown in growth was partly the result of factors outside the government's control, but insisted: "It isn't just about the international environment, it's because of the strategy the government has followed."

Another former Whitehall insider, Vicky Pryce, the head of the government's economic service before becoming a director at FTI Consulting, said that ministers would be advised to begin preparing the ground for a U-turn: "It's a very risky situation, and I think that at some stage they'll panic."

Professor John Muellbauer of Oxford University, an expert on the housing market who signed the letter to the Sunday Times last year supporting the Conservatives' approach, said: "Things are going badly. I had hoped that the focus in the budget would be on improving growth in the places where there are growth prospects, and also maintaining infrastructure investment, and that they would tackle failures of planning."

Tim Besley, a former Bank of England monetary policy committee member who orchestrated the letter, said: "Everybody has been disappointed with growth." He still believes that the chancellor must stick to his guns, but complained that the Treasury had failed to articulate where growth would come from in the months ahead. "The disappointing growth has almost nothing to do with the fiscal plans of the government – I'm coming to the view that it's just a long, slow, hard slog – but what I would like to see from the government is a much more clearly defined growth strategy." Danny Blanchflower, another former MPC member, said: "Economic policy is in disarray."

The increasingly bleak prospects for recovery have also prompted more than 50 prominent leftwing academics to write to the Observer to demand that the government pursues a plan B, to boost jobs and growth. The signatories – including Sir Tony Atkinson and David Marquand of Oxford University, Marcus Miller of Warwick and Richard Grayson of Goldsmiths in London, a former Liberal Democrat policy director – say that if Osborne keeps to his policy, there will be "a lot more pain and a lot less gain". They call for a crackdown on tax evasion, a targeted industrial policy – including investment in green technologies – and higher taxes on the rich to create jobs and growth, saying, "these are the foundations of a real alternative".

A Treasury spokesman said: "We haven't seen anything that makes us question what we are doing." The political argument over who can be trusted with the economy has intensified in recent days. Osborne's predecessor, Alistair Darling, accused him on Radio 4's Today programme of being "mendacious" in insisting that the government had to slash spending or risk a Greek-style meltdown.

Ed Balls, the shadow chancellor, said: "The disappointing figures we've had, particularly on manufacturing, seem to be further evidence that the economic recovery is stalling. We're now set for slower growth, higher inflation and higher unemployment than was forecast a year ago. And the result is that the government is now set to borrow £46bn more than they had planned."

Ivan Lewis, the shadow culture secretary, writing on the Observer's Comment is Free website, says that Labour needs a credible economic and fiscal strategy of its own. He concedes that the party did not spend "every pound wisely", but insists that the deficit was caused by the global banking crisis, rather than spending.