There are many people who believe gym membership alone will make them fit. Similarly, the Labour government seems to think that signing up to a framework of fiscal rules automatically brings budgetary prudence. Yet, in recent years, the government has shown less enthusiasm for making its spending meet its rules than making its rules meet its spending. That is like worrying more about paying the membership fees than spending time on the exercise bike. And now, however, the looming downturn and consequent deterioration in the public finances are set to expose the failings. But fiscal discipline is vital so new rules are needed.

The current rules are designed to enforce two principles. First, future generations should not pay for today's running costs. The "golden rule" states that, measured over an economic cycle, current spending should be met from current taxation. The Treasury should borrow only to build new roads, schools and hospitals. Second, the government should not leave ever-increasing debt repayments for future generations; the "sustainable investment rule" insists that net debt must not be higher than 40 per cent of gross domestic product.

These are good principles, but bad rules. The rules allow revisions to growth in earlier years to alter the apparent health of the public finances today. But past virtue is not an excuse for current bad behaviour. The rules should look forwards, not backwards. A possible revision of the golden rule would be to target a balance in the cyclically-adjusted budget by a fixed date.

Again, the sustainable investment rule considers only the net debt position, but not the value of what is being bought. Measuring liabilities without paying any attention to assets is absurd. This has become more obvious as the government has come closer to its limits.

Yet two difficulties arise with any reforms. One is that the more complex the rules the more difficult they would be for the public to understand. Another is that if rules are changed whenever they are not met, the less credible any rules become: marriage vows are less credible at one's second wedding. Just changing the rules would be a big mistake. The institutional arrangements must also change.

As defendant, judge and jury, the Treasury decides whether it is meeting its own rules. In practice, not surprisingly, it meets them in letter, but not in spirit. Indeed, the Treasury has encouraged departments to circumvent the framework with private finance initiatives.

A better approach would be to establish a new body, to check that the fiscal position met a set of broad principles, such as that each generation paid its own way. The institution should be answerable to parliament, like the National Audit Office. To reduce distortions, it should keep all categories of government debt under review, from public pension liabilities to private finance initiatives.

The system might be modelled on the "Ken and Eddie Show". This was the double act, during the last Conservative government, in which Kenneth Clarke, then chancellor of the exchequer, was given public advice on monetary policy by Eddie George, then governor of the Bank of England. The body should say what state it believes the public finances should be in, assess the probability of meeting those criteria and suggest policy changes. The government could choose to comply with these suggestions or explain why it ignores them.

The government might reform the current framework merely to avoid admitting that it is breaching its rules. But it should bring in a better system to ensure that it spends effectively, invests adequately and remains solvent, without crowding out private activity. In an ideal world chancellors would not need to be forced to act prudently. But, sometimes, the only way to get reluctant exercisers to follow a fitness regime is for someone else to keep a harsh eye on their progress.