When David Cameron used the nuclear option in the early hours of Friday morning, and incensed Angela Merkel and Nicolas Sarkozy by vetoing a new Europe-wide treaty, he claimed to be defending the British economy.
Yet it was hard to find many business voices supporting the decision this weekend. Sir Martin Sorrell, boss of the multinational advertising group WPP, summed up the concerns of many business leaders when he told the Observer: "Intuitively, it can't be helpful. I'd rather be inside the tent." He added that the spat between Britain and the eurozone countries was reminiscent of the kind of internal politics he has had to deal with: "It reminds me of the battles that go on inside companies in our industry between country managers and regional headquarters – it's really a political battle over power and control."
Manufacturers, whose fortunes lie at the heart of the Cameron-Osborne plan for revitalising the economy, and many of whom are heavily reliant on demand from the EU, also express caution. Steve Coventry, head of government affairs at manufacturers' association the EEF, says: "What we need to do is step back, forget about the politics of this, and think about the practicalities of the way it's going to affect how we engage with our EU partners on a daily basis."
In theory, decisions about trade rules, financial regulation and so on are meant to form part of the architecture of the single market, and therefore be decided by all 27 member states; but industry fears that, over time, the core everyone-but-Britain group will inevitably shape the direction of policy.
Julien Seetharamdoo of Coutts plays down the short-term risk of a two-speed Europe leaving Britain on the fringes, but warns foreign direct investment might eventually suffer if Britain distances itself from the EU. "The immediate impact won't be particularly significant; Europe will still be the UK's main destination for exports and we are still part of a free trade area," he says. "But it could have an impact on the degree to which foreign companies will want to invest in the UK. Japan and the US have always viewed the UK as a springboard into Europe."
With relatively few domestic industrial champions, the UK has relied on attracting overseas firms by offering them the advantages of access to the EU single market without the more rigorous regulation of areas such as labour markets of its continental neighbours.
The chancellor's "plan for growth", which he announced in the budget and elaborated on in the autumn statement, involves cutting red tape, reducing corporation tax and helping to fund innovation in areas such as advanced manufacturing, in an effort to invite foreign investment and boost exports so that the UK can pay its way in the world. Nothing in the new "fiscal compact" should immediately jeopardise that; but experts fear that if the rest of the EU hurtles towards closer union, Britain will have to fight to hold on to its place as a full member of the single market.
UK Trade and Investment, the government's trade promotion body, is encouraging British firms to look beyond Europe to emerging markets such as China and India. But about half of UK exports still go to the EU; and in global trade negotiations, it's the EU, not its individual member countries, that gets a seat at the table.
Even in the City, whose interests Cameron was avowedly defending when he flew to Brussels, there were fears that the prime minister had overplayed his hand. A month ago, an unprecedented coalition of all the City's major traders – encompassing banks, hedge funds, stockbrokers and derivatives dealers – warned in an open letter to Osborne of the dire consequences of an EU-wide transaction tax that they feared a new treaty could bring. In some business areas, up to 90% of all transactions could flee London, it said. Yet after the veto, even some of the signatories are privately warning that it could backfire against Britain.
"It doesn't protect us, it exposes us," says one. There was puzzlement, too, because some of the legislation Britain plans to unilaterally impose on its financial sector – the recommendations of the Vickers commission on banking, for example – is actually tougher than anything being mooted at EU level. The Treasury insists that part of Britain's motivation was in fact to ensure that it would be allowed to set its own rules without EU interference.
Nevertheless, a huge amount of financial services legislation is currently coming down the Brussels pipeline, and fears are growing that Britain will no longer be able to influence directives that, despite the veto, will still have full legal force in the UK. Cameron may have drawn a line preventing the EU from imposing a "Robin Hood" tax on financial transactions, but while Britain remains in the EU, it cannot sidestep other EU-wide directives agreed by the commission and passed in the European parliament.
The British Bankers' Association acknowledges that the UK's interests may be harmed by losing its place at the table. BBA chief executive Angela Knight says: "We do not yet know the impact this new arrangement is going to have on the UK's ability to secure agreements on sensible regulation – but that is critical." And the Association of British Insurers adds: "The immediate challenge for us will be exerting influence over EU regulations that will affect the UK financial services industry and its customers."
The insurance industry is currently undergoing intense negotiations in Brussels over "Solvency II", a directive to be introduced next year aimed at increasing the amount of capital insurers must hold. Critics warn it could result in higher premiums and that some UK-based insurers may quit for offshore locations.
But Britain's day-to-day engagement with the EU may suffer much less than people imagine, says Richard Saunders, chief executive of the Investment Management Association. "At the technical level – the level at which things are discussed in committees and directives are hammered out – I'm not sure there will be a huge amount of difference. I don't think we will be isolated."
In financial markets, the mood was muted on Friday, with the FTSE edging a little higher and sterling slipping just a little, despite what foreign policy experts regard as an historic turning point in Britain's relationship with Europe. Saunders believes markets are awaiting more details of just how tax harmonisation and fiscal union in the eurozone may work before delivering their verdict.
Committed Eurosceptics such as Mark Dampier, investment chief at financial adviser Hargreaves Lansdown (itself a component of the FTSE 100 index) are among those who unreservedly applaud Cameron's veto; he is only concerned about a future U-turn. "I'm glad Cameron had the courage to turn around and say no," Dampier says. "The City still earns a shedload of money for Britain, despite what many people think, and it's important that it is protected."
It is still far from clear whether last week's summit has succeeded in safeguarding the single currency. If the euro is in a slow-motion train crash, and eventually collapses in a year or two's time, Cameron will be able to claim he has done the right thing – but with Britain's economic fortunes so closely intertwined with Europe's, nothing he could do would drag us clear of the wreckage.