Watergate involved plumbers. The expenses of MPs revealed yesterday involved plumbers. But there the similarity ends.

There is something marvellously British about a political imbroglio that has, at its heart, a bath plug charged to the taxpayer and the receipts for the Prime Minister's cleaning bill. It is fantastically Pooterish. If it did not threaten the legitimacy of Parliament, it might almost be funny. As it is, it is the opposite of funny. It is a serious calamity.

When the MPs whose details were revealed yesterday were first elected, they were offered a relatively modest professional salary and an allowance to help with the additional costs of the job. There is an important distinction between an allowance and expenses. Each item of expenses needs to be essential to the job and might be judged individually on the contribution that it makes to the governance of Britain. An allowance provides the recipient with a wide degree of latitude about the costs he or she chooses to defray with it.

Individual items charged out of such an allowance are bound to look ridiculous and unnecessary if printed in a newspaper. The moral outrage against the MPs is understandable but unattractively pious.

To begin with, it is hypocritical. There is not one person who has, say, bought lunch for a potential client who would not look absurd and slightly dodgy if it were printed in the newspaper that they charged their employer for a mango sorbet. There is not a single commercial expenses scheme that, if exposed to scrutiny, would not appear to have ambiguous rules or poor policing. Nor are there many who, if offered a cost allowance as part of their remuneration, would not spend it as the rules allowed.

Of course the suggestion is that MPs - free to set their own pay and conditions - have established for themselves a remarkable package with rules that allow them to line their pockets at public expense. This is simply untrue. And obviously so. Parliamentary rules are outdated, the parliamentary authorities weak and the need for reform clear. But becoming a British MP in order to get rich is a scheme that only an idiot would devise. Providing parliamentarians with an allowance so that they can have a second home and the means to maintain and decorate it comfortably is entirely reasonable.

The outrage against MPs is not merely pious, it is also dangerous. Who wants to be a politician if you are going to be treated, when you make a claim against allowances and the claim is granted, almost as if you were a crook? Who wants to work for an employer - the taxpayer - who needs you to keep two homes, but is furious and rude when presented with the bill? Who wants to let their partner work late nights and weekends just to see them pilloried as lazy and on the take? Who wants to work for an institution - Parliament - that allows all your correspondence to be sold to the press? What sort of people will enter public life? What sort of Parliament will we have?

The system of cost allowances was designed for a different parliamentary age. The Commons does not sit all night, every night, these days and the costs incurred by MPs is different. The age of gentlemen's agreements has passed in the business world and enforcement in Parliament is outdated. So reform is needed. And the fact that a set of generally unremarkable claims (there were exceptions) has caused such a political fuss shows that reform is needed urgently.

We employ Members of Parliament as our representatives. We should pay them well and, while always being mindful that our generosity is not abused, we should seek to hold them to account for what really matters - their performance as legislators.