Tony Blair has many fine qualities. He can be a brilliant advocate; and he has a gift – although he found its limit – for finding agreement among apparently incompatible interests. His supporters also put forward other qualifications for the post of President of the European Council: he has experience of working with European institutions; as Prime Minister he sought to engage Britain more constructively in the European Union – again within limits; and, whatever the critics might say, he is well known and respected among world leaders.

However, The Independent on Sunday cannot support his undeclared candidacy for the job that is now almost certain to be created. This is not simply a matter of his decision to join the American invasion of Iraq. That was an error of judgement, and an important one. It must count against him in consideration for any leadership position. But the Iraq war also undermines Mr Blair's claim to be a unifying force. The issue itself was divisive, pitting the governments of the European Union against each other. When the choice between Britain's relationship with America and its relationship with the rest of Europe became unfudgeable, Mr Blair chose America, which speaks volumes about his instincts.

Mr Blair rode roughshod over popular opinion across Europe, and misled people at home. He used information selectively to help persuade Cabinet and Parliament of the case for military action. As we say, he was a forceful advocate, sometimes stretching the facts to the utmost in order to make his case. His lawyerly persuasiveness may be useful in presenting Europe's case to the rest of the world, but it is not necessarily the ability that makes for the best chairman of summits of European leaders.

As we report today, this is the view of none other than Sir Stephen Wall. Sir Stephen was Britain's ambassador to the EU for Mr Blair's first three years as Prime Minister. He was then Mr Blair's adviser on Europe until 2004. In that time, he helped to draft the European Constitution that later became the Lisbon Treaty, which created the post of European president. He claims that it was a British idea, but that Mr Blair might one day fill the post "was not in our thoughts at the time". The job was seen then as one with "very constrained" powers, he says, of "co-ordinating the business of the European Council" – the meetings of the leaders of the member states that take place four times a year.

Sir Stephen says that having a high-profile figure as president "is not necessarily a good idea", which, from someone in his position, is damning indeed. Above all, however, the argument against Mr Blair's appointment is that it would be undemocratic.

This newspaper supports the Lisbon Treaty, but sees a terrible missed opportunity in the way it began – as an attempt to impose a constitution on the EU from above. The EU can only be strengthened if its peoples are given their voice. A referendum on the Treaty would have been essential had it proposed significant changes in the EU – as it was, the demand for a referendum was only a cause behind which Europhobes could rally. But Mr Blair is not the choice of the peoples of Europe, and it would weaken the EU if his appointment were to emerge from the secret horse-trading that is now going on behind the scenes.

Mr Blair is not even the choice of the people of Britain. Anyone who thinks it would be good to have a Briton in the job to fight for a "British" pragmatism in Europe should think again: he would provide a focus for Conservative anti-Europe feeling.

Nor should European leaders choose someone simply on the basis that the Americans have heard of him. They should choose someone who would be interested in making the European Union work better in the interests of its peoples, which means that it should work more democratically.

There is, therefore, a strong case that the first President of the European Council should come from one of the smaller member states. "As a unifying signal it should be thought about," says Sir Stephen Wall. More than that, it should be acted on, and Mr Blair should be stopped.