In the crypt of the Pantheon in Paris are buried the ashes of a little banker from Cognac, an indomitable man who transformed Europe. And although dead these 30 years, he transforms Europe still. When the Second World War ended, Jean Monnet conceived a plan to banish war from the Continent. National governments would gradually yield power to supranational institutions. By sheer will and good contacts (his home was a salon for the lions of the US foreign policy establishment), Monnet helped to found Europe’s fledgeling bodies. But more than this, he provided a working method for European integration.

There were two parts to Monnet’s method — maintaining momentum and what he called “changing the context”. Integration would never be allowed to falter and the politics would continuously changed, with one reform being used to make the case for the next. The plan to make Tony Blair president of the European Council would have made Monnet proud.

Because many features of the Lisbon treaty are arcane, it has been easy to overlook its most potent feature. It creates a figurehead for the Union. If Mr Blair is appointed to this position it will become easy to overlook it no longer.

Politics is chemistry, not physics. The stature of the person holding the office is often more significant than the formal powers he or she has. Some of the possible presidency candidates might be small figures, little able to provide leadership. Mr Blair is emphatically not in that category.

If the European Council appoints Mr Blair it will be installing, without an election, an articulate, charismatic leader who will have a colossal impact on global politics. The future of European institutions, the conduct of foreign policy and, without question, the domestic political position of Gordon Brown and David Cameron, all would be changed by Mr Blair’s ascendancy.

This, of course, is the point of his candidacy. Those promoting him doubtless believe he will do a good job, and that Europe needs an Atlanticist free-trader. But they also believe, as Monnet would have done, that the Lisbon treaty’s creation of a presidency is an opportunity. The appointment of a major figure such as Mr Blair can help, they believe, to strengthen Europe’s political identity and pave the way for still further reform.

Mr Blair’s candidacy may be foiled. There are plenty of European statesman who do not want a British president, let alone one with Mr Blair’s attitude to the United States. But the very fact that Mr Blair’s appointment is a possibility makes the case for a British referendum on the Lisbon treaty.

There is plenty in the treaty that is technical, and which would make an eccentric subject for a national vote. It would also be a grave mistake for the United Kingdom to have a debate about its continued membership of the Union, something that it settled in 1975. But the creation of a strong European presidential figurehead, the sort of leader that Mr Blair would be, is a serious constitutional change, not some mere tidying-up exercise. And therefore it should be put to the British people.

The British Government is keen to emphasise that if the Irish vote “yes”, as seems likely, the debate on Lisbon is over because the treaty will have been approved by every government in the continent. Yet this is only the case because the British Government reneged on its promise to hold a referendum. In other words it is showing exasperation at other people’s failure to accept a situation it brought about itself.

Mr Blair may well make an outstanding president of Europe, but his authority would rest on a broken promise. He should not be installed before the British people have had a chance to decide if they like the very idea of his new office.