How long can this go on? Every week Gordon Brown is presented with new numbers, taken from the Treasury’s own documents, showing his government is planning to cut public spending from 2011. Every week he denies it, repeating the mantra that voters have a choice between Labour “investment” and “Tory cuts”. Last week he gave us a phrase that will surely enter the political lexicon: that Labour would achieve “zero per cent growth”.

Commentators compare Mr Brown with Richard Nixon, who acquired the nickname of Tricky Dicky. In fact, the late US president’s dissembling was much more skilful. No 10 advisers are getting uneasy, as are cabinet ministers, at Mr Brown’s inept efforts. He has not yet promoted a horse to the post of consul but his detachment from reality is growing daily.

While the prime minister persists with his fictions about public spending, the Whitehall machine is gearing up for the biggest cuts since Labour was forced to turn to the International Monetary Fund in 1976. Permanent secretaries realise the spending feast of Mr Brown’s chancellorship is turning to famine.

Civil servants know that long after the politicians have gone they will be clearing up the mess. They are aware that the Treasury’s arithmetic implies real cuts in departmental spending of 7% over the period 2011-4; 10% if health is ring-fenced. Their recommendations for deep cuts for an incoming Tory government will be in blue folders. But the same policy prescription would face a re-elected Labour administration, if Mr Brown were to pull off a victory against all the odds.

The Tories, to their credit, have moved on from platitudes about sharing the proceeds of economic growth between tax cuts and public spending increases to a detailed examination of how they can reduce the size of the state. When the architects of Canada’s drastic spending surgery of the 1990s visited London recently, members of the shadow cabinet were keen to listen to what they had to say. Philip Hammond, the shadow chief Treasury secretary, and Francis Maude, shadow Cabinet Office minister, are adopting key elements of the Canadian approach.

This means searching questions about whether the government needs to engage in certain activities or whether these can be abolished or handed to the private sector. It means not just cutting spending, but also changing the culture that state spending is good under almost any circumstances. It means reining back government, rather than running battles between spending fiefdoms.

It worked for Canada, which cut public spending by 20% over three years in the 1990s without significantly damaging public services or inhibiting economic growth. Canada now has a budget deficit a quarter the size of Britain’s, even in the deepest global recession in the postwar era.

Britain’s public spending is about to climb above 50% of gross domestic product, only partly as a result of recession. Government borrowing this year will be an eyewatering £175 billion. For every £4 the government spends, only £3 are raised in taxation. The rest creates a debt burden bequeathed to future generations.

Governments have to live within their means. This is not just about efficiency savings, important as they are. It is about slim-ming Whitehall empires, abolishing unnecessary quangos and getting rid of the nonjobs and irrelevant functions that characterise Mr Brown’s spending legacy. There will be hard choices but they will have to be made, whether by David Cameron or Mr Brown.