Mary Dejevsky: Britain must join the euro – and Cameron is the man to do it

Almost six weeks have passed since Nicolas Sarkozy memorably told David Cameron to shut up. But the French President did not stop there; he also told him why it was time for him to hold his peace. "We are sick of you criticising us and telling us what to do," it was reported. "You say you hate the euro, and now you want to interfere in our meetings."

In so saying, Sarkozy highlighted Cameron's dilemma as acutely as anyone either before or since. For this is exactly what Cameron was trying to do – what he has been trying to do ever since the euro crisis erupted, and what he will attempt to do again today, at the latest "make-or-break" get-together of EU leaders. He wants to keep Britain on the outside of the eurozone, while maintaining a big-power say on what happens inside. As Sarkozy rightly implied, this is an essentially ignoble and dishonest ambition.

So far, though, Cameron has just about been able to hold the line, however unpopular this might have made him elsewhere in Europe. Both he and his Chancellor, George Osborne, have managed at once to insist on the continued wisdom of keeping Britain outside the euro, while demanding a voice among those whose immediate financial fate is truly at stake. But those days of sitting on the fence, of having cake and eating it, are coming to an end – and that could be as soon as today.

Whether the outcome is the plan for amendments to existing EU treaties – the first choice of France and Germany – or a new agreement among the 17 countries of the eurozone, Cameron has to choose. He can accept the treaty amendments, and, in so doing, make it likely that other doubters will accept them. Or he can veto the idea, which would – almost – automatically trigger a eurozone treaty to which Britain would not be party. It is extremely hard to see how, in this event, Cameron – or any successor – would be able to justify any role at all for Britain on the inside. Inner and outer circles, the two-speed Europe so long spoken of, would be the reality.

The past week has seen a ferocious campaign by Conservative Eurosceptics to force Cameron either to reject the Franco-German plan, or to make his acquiescence conditional on a referendum. The concerns they have voiced in public rest on considerations of national sovereignty and national interest. They claim the changes would damage the City of London as a global financial centre, and, with it, the prosperity of the country as a whole. Underlying their objections are doubtless two other strands: any plan cooked up by France and Germany is bound to be bad for Britain; equally, any new plan will sooner or later impose greater EU financial cohesion.

Cameron has carved out for himself some room for manoeuvre. He says he will veto any treaty changes that he deems not to be in the national interest. And his existing commitment to holding a referendum on EU treaty amendments extends only to provisions that transfer more power to Brussels. There is enough space here – just – for the Prime Minister to navigate his way through. But he will need to be brave. The clamour from Tory Eurosceptics will only grow, as will pressure for a referendum. He will need all the political cover that his Coalition with the Europhile Liberal Democrats affords – and all the three years that remain until the next election for the indignation to die down.

But there is another option. Tory Eurosceptics make a lot of noise, and they have one widely shared prejudice that unites them – xenophobic little Englandism, masquerading as the high principle of national sovereignty. But their demands for Cameron to wield his veto on proposed EU treaty amendments contain one fatal flaw. The alternative to treaty amendments, as Cameron, Osborne and pretty much anyone else in government appreciates, is the two-speed Europe that would threaten London's primacy as a financial centre even more. So who is acting against the national interest now?

This could be Cameron's opening. Given the trouble he is going to be in anyway – with his sceptics, if he agrees to the proposed treaty amendments without agreeing to a referendum, and with business, if he effectively accepts Britain's relegation to a lower financial league – he should summarily end the ambiguity of the past year and embrace the only logical solution: Britain's belated entry into the euro.

Between the lines of what Sarkozy, but also leading German politicians sometimes say, can be detected the thought that, if another big economy, ie Britain, had been in the euro from the start, the operation of the currency would have been better policed. The agreed limits on indebtedness might have been kept; Greece, for one, might not have been admitted. That is all history now. But Britain's entry into the euro would, at a stroke, bring more money and – as the regulation of British banks improves – more financial stability into the European Central Bank, to mutual benefit.

Britain would retain, or even enhance, its position as Europe's financial centre – a price that could reasonably be exacted as a condition of entry. Nor need there be complaints – as there were 15 years ago – that sterling would be entering the euro at too high a rate. Thanks to Gordon Brown and the global financial crisis, the pound has been devalued by almost 30 per cent against the euro. Cameron could thus boast that a key requirement set by the former, Labour, government had been met.

Not only does Cameron have all these arguments and more, on his side, but he has the political means to get his way. He is not John Major, teetering on the brink of losing his majority and in hock to his Eurosceptics. With Liberal Democrats and Europhile Labour MPs on his side, he could win a majority in Parliament and campaign country-wide for a referendum "Yes" as the only true representative of the national interest.

With his PR skills and his one-nation Tory credentials, Cameron is one of the few British politicians who could convince mainstream voters to accept the euro. The Republican, Richard Nixon, initiated the US opening to China; the Likud Zionist Ariel Sharon took Israel out of Gaza as the prelude to a Middle East peace (regrettably halted by his illness). David Cameron should be the Conservative who made Britons into Europeans.