Leading article: Ultimately, only an election can drain this political poison

After a week of revelations of MPs expenses, it is time to take stock. There is an angry mood in the country towards parliamentarians – and understandably so. It is clear that a substantial minority of our MPs have engaged in behaviour that would leave most of us facing dismissal at best, possibly even prosecution at worst. Others, while not engaging in fraud, have been guilty of greedily milking the system, breaching the clear spirit of the rules governing allowances.

This much we know. But what is less certain is the damage this business will inflict on our democratic system. At this stage, unfortunately, there are good reasons for fearing that the harm will be deep and lasting. Individual MPs have been embarrassed and their future careers thrown into doubt. But that is nothing set against the damage that has been inflicted on the moral authority of the House of Commons itself.

This was illustrated by the response to yesterday's report from the Treasury Select Committee into bankers' bonuses. One of the report's arguments – that remuneration policies in the financial services sector encouraged dangerous risk-taking during the bubble – was unremarkable. More significant was the reaction the report provoked. What right, it was asked, do MPs have to criticise the pay practices of others after all the evidence of greed in the Commons that has emerged in recent days?

This is merely the beginning. Henceforth, whenever MPs address subjects ranging from benefit fraud, waste in the public sector to taxation, they risk provoking not serious debate, but mocking laughter. The tragedy is that we live in times when such matters have a rare importance. The financial crisis and the collapse of the public finances in the recession have left Britain at an economic crossroads. Yet the expenses scandal seems to have shattered our political signposts.

Some MPs have complained this week of "mob justice". And those with clean hands complain they are suffering from a sort of collective punishment. It is true that there has been a slightly hysterical edge to some of the public opprobrium being heaped on the heads of parliamentarians. An MP who claims for a packet of biscuits as part of his monthly groceries allowance does not deserve the same scorn as an MP who dodges paying tens of thousands pounds of capital gains tax.

Yet the Commons, as a body, deserves little sympathy if it is being harshly judged now, after the shameful manner in which it tried to stop these details from reaching the public domain. The hapless Speaker, Michael Martin, used taxpayers' money to fight a case in the High Court to prevent their publication. And MPs, with some honourable exceptions, sought on several occasions to remove themselves from the reach of the Freedom of Information Act. If MPs had adopted a policy of transparency they could demand a fair hearing now. As it is, they have little choice but to swallow their medicine.

The depressing thing is that despite the flurry of cheques that have been written to the taxpayer in recent days and the promises of reform from the party leaders, some MPs still seem unable to understand the public anger. Even now, they point to the supposed inadequacy of their £64,000 annual salary, as if that was an excuse for fiddling expenses.

This is a significant moment for British politics. It is possible that a wave of public anger will prompt a surge of protest votes for fringe parties in next month's European elections. And if the perception of corruption lingers, a crisis of legitimacy for the established political system cannot be ruled out.

To limit the damage there are certain things that need to happen. The first is for the Speaker to depart. Promises of reform are not credible while the compromised and unrepentant Mr Martin remains as the public face of Parliament. Second, the leaders of the main parties need to remove any MP guilty of abusing the system from their respective frontbenches. David Cameron and Nick Clegg have been far more responsive to the public mood in this crisis than Gordon Brown. But the truth is that all of the mainstream parties have been damaged by this affair. They need to put partisan concerns on one side and begin the construction of a modest and transparent expenses system without delay.

Yet, as this scandal stretches out, it seems ever more likely that only the ballot box can provide the catharsis the body politic requires. Only a fresh general election, giving the public the opportunity to expel those MPs who have abused the system, will restore confidence in our Parliament. For the sake of our democracy, the sooner that purging takes place, the better.