“A step into the unknown” – that was how one eminent retired mandarin described Gordon Brown’s planned “fiscal stimulus”, the predicted £15 billion of tax cuts and increased spending designed to jump-start the British economy.

“It is like being blindfolded and told to walk along a narrow plank across a chasm,” said the former cabinet secretary.

His remarks reflect a wider unease among retired and serving Whitehall officials over the prime minister’s proposed Keynesian borrowing splurge.

Deep concern is emerging among officials charged with implementing Brown’s policy – they fear that, although opening the fiscal tap may offer short-term relief, the longer-term consequences could be highly damaging.

Brown and Alistair Darling, his chancellor, are putting the finishing touches to tomorrow’s pre-budget report.

It has been billed as Labour’s blueprint for economic recovery – and the outcome of the political debate between Brown, the big spender, and David Cameron, the Tory leader who last week spelled out his opposition to unfunded tax cuts, could determine who wins the next general election.

Ministers will tomorrow announce measures to help small businesses and there will also be moves to unblock the property market.

However, the centrepiece of the statement will be the “stimulus” – the Treasury has not disputed economists’ speculation of £15 billion.

There will be help for the low-paid – the £120 temporary increase in basic-rate tax allowances is to be extended.

The low-paid are thought to be more likely to spend the money they receive, thus stimulating the economy, while the better-off are assumed to save more.

There is a political argument too. A few months ago, Brown came close to being toppled by rebels in his own party because of a previous tax change abolishing the 10p rate, which took money from Labour’s core working-class votes.

However, the middle classes are also expected to benefit from Vat cuts – possibly before Christmas. The sales tax reduction is likely to last for up to two years in order to encourage consumers to return to the shops.

A senior Whitehall source said: “This will be a multi-faceted package. We will be addressing the fairness agenda, but it is important the whole country benefits from any stimulus.”

On the spending side, a thousand Keynesian “holes in the ground” will be dug. Geoff Hoon, the transport secretary, says dozens of modest road improvement schemes will be brought forward.

Among other big initiatives, Ed Balls, the schools secretary, is expected to announce that hundreds of primary schools will be rebuilt sooner than expected.

Whitehall’s wise men are worried, however. Lord Turnbull, who was permanent secretary to the Treasury before becoming cabinet secretary from 2002 to 2005, warned Brown against announcing a large fiscal stimulus.

“We need to approach this with a lot of caution,” he said. Turnbull pointed out that Britain was in a worse position because it had started the downturn with a budget deficit, forecast to rise to £90 billion next year and £120 billion in 2010.

“Because of our budgetary position where our imbalance is larger than other countries’, by definition we are able to do less,” he said.

Officials fear tax cuts or spending measures announced as temporary may prove permanent. “We have to make sure we are not building in long-term costs,” said Turnbull.

His remarks echo those of Lord Burns, another former permanent secretary to the Treasury, who has spoken out about the risks of unfunded tax cuts.

Their statements will be helpful to Cameron, who has struggled to find allies for his policy of fiscal rectitude. Most business groups and economists believe a stimulus of up to £30 billion is vital.

The Conservative leader last week signalled that public sector budgets may be cut in real terms if his party wins a general election in 2010.

In an interview with The Sunday Times, Cameron makes one of his strongest attacks yet on Brown’s economic record: “He knows he has a huge share of the responsibility for the mess we’re already in. He knows it’s going to get worse and I think he knows the longer this goes on the more he’s going to get found out.

Cameron has the support of a “brains trust” of former Tory chancellors. His policy was formulated after a Commons breakfast with Kenneth Clarke, Lord Lamont and Lord Howe.

The Conservatives yesterday unveiled a campaign poster modelled on the 1992 slogan of “Labour’s tax bombshell”. Labour will hit back by warning of “Tory cuts” to essential public services, a line of attack it has used since the 1980s.

The politics of the credit crunch may have turned retro. However, the debate will determine how Britain survives the recession – and which party dominates the next decade.

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