When the likely date for the referendum on electoral reform was leaked by Nick Clegg as 5 May next year, I immediately posed the "French Ambassador has died" question. (Equerry to British ambassador: "Your excellency, I regret to inform you that the French ambassador died during the night." British ambassador: "I wonder why he did that?") I suspect it was to end the silence between the coalition parties and to flush out some reactions.

Rumours that Conservative MPs had been asked not to discuss PR were circulating some time ago, but I understand that the Electoral Reform Society and senior Liberal Democrats have concluded that the alternative vote option presented in the coalition agreement is the best that can be achieved at this stage and that any discussion on the issue will cloud the debate.

Now that Boris Johnson is also reported to be opposed to first past the post (FPTP) for Westminster and favours coalition politics things are livening-up.

The recent election was the most sophisticated in my experience. Tactical voting was widely used. Even though turnout in the European elections is low, voters throughout the UK are now familiar with PR as a system. Most people actually like the coalition.

It is to the credit of the Conservative leadership that it pledged to reform the debased "closed regional list" PR system for Euro elections in last June's manifesto. Now is the time, with a wholly new political landscape, to consider whether the wording of the coalition agreement could embrace a third choice on the referendum ballot paper – the single transferable vote (STV).

The agreement, be it noted, "includes provision for the introduction of the alternative vote…" which does not rule out alternative propositions being put:

"The parties will bring forward a referendum bill on electoral reform, which includes provision for the introduction of the alternative vote in the event of a positive result in the referendum, as well as for the creation of fewer and more equal sized constituencies. Both parties will whip their parliamentary parties in both houses to support a simple majority referendum on the alternative vote, without prejudice to the positions parties will take during such a referendum."

It is not my purpose to argue the case here for STV. However it is the most widely used system worldwide – and it is also used for the election of the Speaker of the House of Commons and was recommended in 2005 for elections to the House of Lords. Supporting that measure were, among others, William Hague, Sir George Young and Andrew Tyrie.

STV, the preferred choice of the Electoral Reform Society, is the best form of PR because every vote counts. It has been used for Euro-elections in Northern Ireland since 1979 to ensure a balanced result. It is also used in the Republic of Ireland to elect their MEPs. I have observed over the years that Irish MEPs are the most assiduous constituency workers. Last week, the LSE published a survey of MEPs' activities since the Euro-election and Irish MEPs are streets ahead of the other 26 member states. One of my responsibilities is to chair Question Time: it is a very Irish affair.

STV was proposed for Westminster by the House of Commons in 1917 but rejected by the House of Lords. STV uses preferential voting in multi-member constituencies. Each voter gets one vote, which can transfer from their first preference to their second preference and so on, as necessary. Candidates don't need a majority of votes to be elected, just a known "quota", or share of the votes, determined by the size of the electorate and the number of positions to be filled.

I do not believe that STV is too complicated for the British people to understand. While leader of the Conservative MEPs, I "won" the first national election under PR, the 1999 Euro-election, although the main credit should go to William Hague, then leader of the Conservative party, for the slogan "In Europe, not run by Europe". We trained the Conservative party to recognise the advantages of a system which made every vote count: the "Heineken effect" which reached parts of the electorate which FPTP does not.

I have long been a supporter of proportional representation and organised a well-attended fringe meeting at last year's Conservative Spring Conference. After my expulsion from the party for challenging David Cameron's choice of European allies, I joined the Lib Dems in March and regard the coalition as the best outcome for Britain.

I am convinced that Britain's political establishment should have the maturity to allow a debate to begin about the unspoken third choice. On 21 July, jointly with Conservative Action For Electoral Reform , I am organising a Westminster event at which Keith Best, its president, and I will be joined by the Tory MP Douglas Carswell – the MP who still argues for a referendum on the Lisbon treaty.

For me this is a point of principle. After the fall of the Berlin Wall I founded the €140m EU Democracy Initiative. This promotes democracy worldwide and I believe we should have it at home too. Single party advantage has no part to play in what amounts to a change of constitutional significance. Westminster has been in a mire and is too self-regarding. Over the weekend, reading the commentators online in print and in the media the consensus is for real change, not partisan politics. The best is not the enemy of the good; the best is best and the best is STV.

Populists such as Douglas Carswell want referendums on a wide range of topics. I do not, but we are united on this point of principle, that the British people deserve a real choice, that the case can be made for including an STV option in the bill which will no doubt be timetabled by Nick Clegg next week and that it will be voted for by a majority in the referendum.