The Lib Dems missed a golden opportunity genuinely to re-shape politics in this country by failing to negotiate seriously with us on constitutional reform.

My colleague Andrew Adonis has already written about their weird proposal that the alternative vote be introduced without a referendum, but they also failed to push for the "plus" on the end of AV that would produce proportional representation.

This leaves Nick Clegg pontificating about "the biggest shake-up of our democracy since 1832" in a speech that offered no protection for the Human Rights Act and made no mention of the genuine electoral reform that his party was supposed to champion.

It is time for Labour to end the ambivalence that prevented us from honouring our 1997 manifesto commitment to a referendum, offering the British people a choice between the current system or a proportional alternative.

In the late 90s, it was difficult trying to convince some colleagues to change the electoral system that had given us a landslide majority and it's fair to say that there was no sense that the public wanted change.

In addition, many of my colleagues argued outcome rather than process. They recognised that the first-past-the-post system (FPTP) forced voters to choose between the party they wanted to run the country and the person they wanted to represent them. They accepted that, in large swaths of the country, votes were wasted as safe seats rarely changed hands. They appreciated that the battle of political ideas at election time was focused exclusively on a minority of swing voters in marginal seats.

But they argued that FPTP produces a clear outcome and strong, single party government.

Does anyone doubt that, between the expenses scandal and the outcome of the general election, the landscape has utterly changed? And changed for ever. Contemplate the outcome of this general election as you read the words of David Cameron in the Evening Standard last year: "The coalition governments [PR] inevitably creates, inevitably descend into backroom deals that betray the will of the people. Instead of voters choosing their government on the basis of the manifestos put before them in an election, party managers put together a government that suits them after rounds of horse-trading and bargaining for power."

An accurate description of what was to come under a voting system that was supposed to avoid it.

So what now? The Labour leadership election has spawned much necessary internal contemplation and will produce more rhetoric about empowering the electorate. Here's how we turn the platitudes into policy.

The new government is committed to a referendum on a new voting system. It will contain two options — the current first-past-the-post system and the alternative vote. It will be the first time in the history of our democracy that its citizens will have a say in how their votes are translated into political power. What possible argument can there be against adding the recommendation of the Independent Commission on the Voting System, AV+ as a third option? It retains the constituency link, extends voter choice and is broadly proportional.

I hope that all the opposition parties, backed by a popular movement throughout the country, unite to press the Tory-Lib Dem government to give the public the option of genuine electoral reform in a referendum. If not, I will certainly be making the case within my own party to submit legislative amendments to that effect.

Alan Johnson is MP for Kingston-upon-Hull West and Hessle