David Cameron urged Angela Merkel on Friday to help keep Britain in the EU, amid concerns in senior Tory circles that many hedge fund chiefs will throw their financial muscle behind the "Brexit" campaign.
The UK prime minister entertained the German chancellor at Chequers with a dinner intended to accelerate his EU renegotiation, which is making slow progress in Brussels.
Mr Cameron's allies fear that the longer the talks drag on, the easier it will be for Brexit campaigners to make the political running, partly funded by eurosceptic hedge fund executives.
While several hedge fund managers have been big donors to the Tories, Crispin Odey, a leading hedge fund manager, supports the "Vote Leave" Brexit campaign, launched on Thursday night. "The hedge funds are resolutely anti-EU," said one leader of the pro-EU campaign.
Mr Cameron expects much of corporate Britain to support Britain's continued membership of the EU.
The official campaign for Britain to stay in the EU is bringing in corporate heavy hitters, announcing on Friday night that it will be chaired by the Conservative peer Lord Rose, the man who turned around the fortunes of Marks and Spencer during his spell in charge of the retailer.
The cross-party "In" campaign, to be launched on Monday, features New Labour grandees including Lord Sainsbury, its biggest donor, Lord Mandelson and the public relations chief Roland Rudd.
But allies say Mr Cameron will have to deliver a good renegotiation to win over many in London's hedge fund sector.
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The sector's hostility partly stems from the battle over a post-financial crisis Brussels directive that brought hedge funds and private equity groups under direct regulation for the first time.
One prominent hedge fund manager said potential donors to the Brexit campaign were not yet decided and that few would voice their views publicly. "There is a division of opinion over the referendum," the manager said.
Sir Michael Hintze, founder of the hedge fund CQS, gave £1.8m to the Tories between 2010 and 2015, making him its fourth biggest donor over the period. Another, Chris Rokos, a former Brevan Howard trader, gave £1m over the same period. Sir Michael has been publicly critical of the effect of the EU's regulation of the hedge fund industry, arguing that it has made it more difficult for hedge funds to raise money in Europe. CQS declined to comment.
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Both sides of the argument can spend unlimited funds until the official campaign begins, three to four months before the in-out referendum; at that point both "designated campaigns" have a spending cap of £7m.
The In camp recognises it needs to step up fundraising to avoid being outspent during the phoney war. "We are confident that hedge funds and many other financial firms will support 'Vote Leave, take control'," said Paul Stephenson, a spokesman for the campaign group.
Amid claims and counter claims, Andrew Tyrie, chairman of the Commons Treasury committee, has announced that his body will investigate the costs and benefits of EU membership.
MPs will take evidence from a range of witnesses, expected to include Mark Carney, the Bank of England governor. It will also examine the "impact of departure".