It is considered a golden rule in politics that Budgets that look good on the day, start to look poor by the weekend and vice versa. This week's Budget has broken the rule. It didn't look very good on the day. Now it looks worse.

Yesterday's shocking GDP figures, recording a contraction far worse than was expected, make clear that even the dreadful borrowing figures announced by the Chancellor of the Exchequer were not dreadful enough to describe the reality.

Espying the Prime Minister's glowering countenance, it is hard to advance the hypothesis that a natural tendency to be optimistic is the cause for these rosy forecasts. Nor is Alistair Darling normally to be found skipping along, singing Bring Me Sunshine. A more credible conclusion is that the Government implicitly understood the consequences of being more downbeat about the future of the economy. And having understood them, it chose not deal with them.

One politician accusing another of dishonesty is usually unconvincing and unedifying. Voters are tempted to react like the weary parent presented with two squabbling siblings, each insisting that the other started it. But when the Conservative Party decided to describe this Budget as lacking honesty, they were correct.

The dishonesty takes three forms. First, the assumption that this is an ordinary cyclical downturn, from which the country will bounce back as it has from other downturns, is untenable. There will be deep structural effects. Since the Prime Minister devotes a considerable amount of time to explaining why, in his view, the country has not returned to ordinary boom and bust, it is odd that he allowed forecasts to be issued that embody precisely that idea.

Even if the political temptation of saddling a future government with the job of revising the forecasts is obvious, the Treasury should surely have argued vigorously for a more realistic set of predictions.

The second form of dishonesty relates to the income assumed to come from raising tax from top earners. Again, the political temptation of booking large sums from these rises and challenging the Conservatives to dispute them is obvious. But the amounts assume there is little change in behaviour, and that top earners will go on earning in the same way even when they are no longer benefiting in the same way. This would be a heroic assumption if its motivation wasn't so unheroic.

The Institute for Fiscal Studies suggests, based on previous experience, that once the accountants get busy the top-rate tax rise will not raise the large sums the Government posits. And the hidden rise in tax on the pensions of top earners, revealed by The Times on Thursday, may prove hard to make work. Once they have served their purpose in the political game, therefore, a future chancellor will have to seek other measures to secure the revenue.

Finally, and most importantly, the Budget was dishonest because it failed to show how the vast amount of borrowing would be repaid. The subject of the cost of £140 billion of structural debt was regarded as too delicate to raise. But by next year the Treasury expects debt interest payments to equal expenditure on education and defence combined.

So the Conservative charge of dishonesty has force. However, it also has an important implication. If the Tories are to make this charge then they must be scrupulously honest themselves. They must be clear that they intend to restructure the State and they must give a clearer indication of how they will do it. Talk of efficiency savings and scrapping ID cards is not enough. If David Cameron says that it is time for a change, he must tell us - change to what?