Unambitious deals deliver limited economic benefits but are easier to agree on. An ambitious deal can bring major economic benefits but comes with political risks, including the risk that the negotiation breaks down. Sometimes ambitious goals are hollowed out during the negotiations.

On other occasions, though, an ambitious deal may be the only realistic option: In the EU’s ongoing efforts to reach a free-trade deal with Japan, for instance, Japan’s primary objective is significantly easier access to the European car market—something the EU could only trade in return for a very good deal on access to protected Japanese markets such as food and public procurement.

In the case of the EU, British Prime Minister Theresa May has already signaled that she will be seeking the deepest and most comprehensive free-trade deal ever contemplated—one that replicates as much as possible the frictionless trade in goods and services that the U.K. currently enjoys as a member of the EU’s internal market. In the case of the U.S., the prime minister has yet to say what she wants from the free-trade deal that she plans to discuss with President Donald Trump in Washington this week. But any U.S. deal will likely need to be ambitious too.

That is because both negotiations are likely to hinge on financial services. The U.K. is certain to want to secure the greatest possible access for its most valuable export industry to the EU and U.S., cementing London’s status as the world’s leading financial center. But achieving this won’t be easy, notwithstanding that the City of London is already the EU’s financial center.

To see why, consider the tricky question of the regulation of clearing houses, likely to be one of the main flashpoints in the EU negotiation. In the post-financial crisis world, clearing houses sit in the middle of the vast majority of securities transactions, acting as a central counterparty for both sides. The sums flowing through these private institutions are colossal: some €10 trillion ($10.7 trillion) of euro-denominated interest rate swaps are cleared through London every year, equivalent to 10 times the eurozone’s gross domestic product.

Eurozone policy makers have long been uncomfortable that such vital financial infrastructure is located offshore, not least because they would need to provide liquidity to prevent the system from collapsing despite having no formal legal or regulatory powers. Post-Brexit, eurozone policy makers will have even less legal powers because the U.K. will no longer be subject to the jurisdiction of the European Court of Justice.

That raises the question of whether the eurozone will continue to allow firms under its supervision to use London-based clearing houses. The UK’s best bet may be to seek an “equivalence” deal based on mutual recognition of each other’s regulatory standards, according to a paper published this week by the International Regulatory Strategy Group, a London-based industry group.

But it isn’t clear whether this would be acceptable to the eurozone, given the size of the offshore exposure or whether any strings it might attach to such a deal—such as binding commitments to follow EU rules, shared oversight, continued jurisdiction of the European Court of Justice—would be acceptable to the U.K.

Similar concerns are likely to loom large over efforts to deepen British access to the U.S. financial-services markets. This was the U.K.’s primary objective for the ill-fated EU-U.S. Trans-Atlantic Trade and Investment Partnership. On that occasion, it ran into strong resistance from the U.S. Treasury, which argued that the federal government had no authority to bind the hands of independent U.S. regulators. Is Mr. Trump willing to go further than the Obama administration and order U.S. regulators to allow British-based firms to sell services across the U.S.? And as with the EU, what oversight of U.K. regulation might U.S. authorities demand in return?

To make the U.K.’s task harder, it is seeking to negotiate these new arrangements just as deep divisions are opening up between the U.S. and EU over the rules governing the global financial system. For the last eight years, there has been a global push to deliver global financial rules that protects national taxpayers but allow international capital to keep flowing.

But now, just as that process is coming toward the end, the U.S. and some European countries are at loggerheads over the latest proposals to overhaul the way capital requirements are calculated: U.S. policy makers accuse European banks of gaming the current rules, while European banks complain that the new rules are being designed to favor their U.S. competitors. Meanwhile, President Trump has said he wants to rewrite financial rules, which he says currently restrict the flow of credit.

Failure to reach agreement on the final package of so-called Basel rules in the coming weeks could open the doors to “currency nationalism” as countries make their own rules and financial globalization goes into reverse, one senior British policy maker warns.

That hardly bodes well for a country whose ambitions hinge on putting itself at heart of the global financial system. But with high ambition comes higher risk.