In a wood-panelled private dining room in Knightsbridge, a dozen refined Swedish men gathered for dinner earlier this week. They all live in London and, for the first half of the evening, they spoke in barely accented English about the business environment in Britain. The conversation was high-brow, if a little bloodless.

Then the former chairman of a landmark British business raised the subject of nondoms. The polite salon atmosphere evaporated. Everyone around the table, it turned out, was a non-domiciled resident of the UK. Each of them was appalled by the current proposals to impose new taxes on non-doms. And half the people there said they were considering leaving the UK before the new nondoms taxes come into force next April. One described himself as having “fled” the tax regime in his native Sweden. Another described himself as having found “financial asylum” in this country.

Another explained, a little more soberly, that the Government’s planned levy of £30,000 on each nondom is more of a disincentive to live in the UK than it first seems: a non-dom couple, for example, would have to pay £60,000 extra per year, which is the equivalent of being taxed on the annual income gained on a cautiously invested £4 million.

It is all very well to focus on the likes of Roman Abramovich and Lakshmi Mittal when talking about non-doms. No doubt the new levy is petty cash to billionaires. But for the average, cosmopolitan millionaire - the forgotten middle class of the non-dom world – the new levy is a meaningful sum. It also constitutes a worrying change in the nondom tax rules that many fear will be just the beginning of a gradual increase in taxation on foreigners in Britain.

The fuming Swedes around the dinner table said that they all already paid tax on their UK income – the current non-dom rules tax foreign citizens based in the UK on what they earn here but do not tax international earnings and capital.

They warned, therefore, that the Treasury is in danger of losing out on hundreds of millions of pounds, maybe more, in non-dom revenue. More important still, a raft of talent will drift out of the country.

George Osborne’s idea of all but abolishing inheritance tax and paying for it by taxing non-doms has been widely applauded. No wonder. Who is going to boo a proposal that cuts the tax bill for British voters and places the burden on the shoulders of a bunch of unknown, wealthy foreigners? A week later Alistair Darling’s proposals to scrap taper relief on capital gains tax has stirred up the fury of small business but the plan to tax nondoms met with barely any public resistance.

The problem is that, as things stand, it is impossible to know whether or not the Swedes around the dinner table in Knightsbridge were just engaged in special pleading or sounding a genuine alarm. That is because the Government has gathered scant information about nondoms. Worse, the data that it has collected has been made public in a patchy and unhelpful way.

A serious change in the treatment of a small but important sector of society is under way. Its impact needs to be carefully considered and debated in view of the facts. The Government’s first task should be to collect and publish them.

The tax code needs to be fair and seen to be fair. At the same time, it helps no one if it depresses consumption, drives away wealth creation and damages the economy. Non-doms are unlikely to be a popular cause but they sorely deserve a public hearing.

Times Recommends

• Lies and cover-up at MG Rover exposed by report
• High oil prices push manufacturing costs up
• Primark founder steps down as chief executive