This is the last chance. This crisis of public trust in parliament gives Labour, in its last gasp, the opportunity to do what it should have done in its first flush of victory – call for a referendum on changing our fossilised first-past-the-post-voting system. Give voters the chance to choose a modern proportional system. Labour must now add a long-promised referendum on the voting system to the ballot paper at the next general election. If this government doesn’t, a Conservative one certainly never will, so it’s now or never.

Tony Blair backed off electoral reform, despite commissioning Roy Jenkins to draw up a moderate and simple PR system, despite dangling the offer in front of Paddy Ashdown. Blair backed off when confronted with the look on John Prescott’s face and the sheer self-interested tribalism of most of his party. He could arm-twist them to vote for war against their better judgement, but not to put their own safe seats in peril.

Everything has changed. Not many Labour seats look safe and the future looks bleak. Remorse at all the reforms that have not been done ought to oblige MPs to offer the voters the chance to vote for change. It is monstrous that no new movement can start up with any hope of winning a seat, crushed by the old parties’ monopoly on power. That’s impossible to justify, while pretending to urge more “community” activism and less political apathy. The two old parties are exceedingly unpopular: ask the pollsters and even before this expenses scandal people voiced intense dislike of both main parties, their whips and their inertly obedient backbenchers.

Listen to the pompous arguments for the status quo: they sound exactly like what they are – a self-interested establishment holding tight to its grip on power. Here’s what they say: because PR means coalitions between smaller parties that means no more “strong government”. But “strong” often means the ability to force through unpopular policies against the will of most people. The poll tax and the Iraq war ended up destroying “strong” governments. The consensus that Mrs Thatcher railed against is usually the wisdom of crowds. Coalitions may be hard to run, but their collective judgements have usually provided better, more stable government in Germany and most of Europe.

Besides, we already have government by coalition, the de facto coalitions that travel under the flag of convenience of the two main parties. Better by far to have, for example, a left of Labour, a New Labour party and probably the Lib Dems and Greens in a coalition, doing their horse trading in public according to their relative strengths as expressed by people’s votes. That way you can’t get one faction seizing the whole party.

The game is up. The mob is at the gates. Tinkering with the expenses system is not enough to send them away again. All kinds of wild and anarchic proposals for a parliament of independents are emerging from an angry public – see the Guardian’s Comment is Free postings. Labour people who used to oppose PR need to seize it with both hands before it’s too late. If not, when they look back from the opposition benches, they will regret it deeply, profoundly and probably for many years to come.