Under European Union law, Greece cannot leave the euro.

That is the theory. But in practice, any protection the law offers investors could be difficult to enforce, according to lawyers trying to protect their corporate clients against the upheaval sure to follow if Greece defaults on its debts and adopts a new currency. So their advice is blunt: Remove cash and other liquid assets from Greece and prepare to take a short-term hit on any other investments. “My personal view is that it is irrational for anyone, whether a corporation or an individual, to be leaving money in Greek financial institutions, so long as there is a credible prospect of a euro zone exit,” said Ian Clark, a partner in London for White & Case, a global law firm that has a team of 10 attorneys focusing on the issue. Several multinational corporations have already taken the same view. Vodafone, the mobile phone operator, and GlaxoSmithKline, the pharmaceuticals firm, say they are “sweeping” money out of Greece and into British banks each evening. This applies not just to Greece but to most other euro nations, although Glaxo says it still keeps money in Germany. Corporate attorneys say looking to E.U. law provides only approximate guidance on whether Greece could stop using the euro while remaining in the Union. Although the E.U. prides itself on basing decisions on strict interpretation of the legal texts in its governing treaty and other legislation, the rules on euro membership have proved flexible. For example, while all 27 E.U. nations are supposedly obliged to join the single currency, once they meet certain economic criteria, Britain and Denmark were able to negotiate the option of retaining their own currencies. Sweden is one of the nations technically obliged to join the euro, but since a national referendum opposed the idea in 2003, no one has pressed the country to do so. Similarly, while leaving the euro might, legally, mean quitting the union itself, most experts see this as a technicality that can be circumvented as well. “The treaty doesn’t cover the question of what would happen if a country were to leave the euro and return to its previous currency,” said Stephen Weatherill, Jacques Delors Professor of European Law at Oxford University. “In the absence of any provision, there is plenty of space for European governments to concoct a solution, adopt it and for it to be legally enforceable,” he added. “In general, you can do anything you like, so long as you do not breach pre-existing international obligations.” The mechanics of leaving the euro would surely lead Greece to impose so-called capital controls to stem the flight of money from a currency destined to be devalued. Again, such controls look impossible under E.U. law. But Mr. Weatherill thinks that a loophole allowing for the protection of public security could be invoked. Mr. Clark, of White & Case, a global law firm, points to a clause in Article 65 of the treaty that says that the pledge on free movement should not prevent countries from taking measures “which are justified on grounds of public policy or public security.” Mr. Clark and his team serve clients that include financial institutions like BNP Paribas and hedge funds. In February, Andrew Witty, the chief executive of GlaxoSmithKline, said: “We don’t leave any cash in most European countries” except Germany. Tens of millions of pounds flow into accounts in Britain every day, he said. But, apart from trying to ensure that debts are paid promptly and therefore in euros, legal options for companies are limited. Contracts covered by Greek law, particularly for services delivered in Greece, provide little protection against the currency’s being redenominated and devalued — a development regarded as unlikely until recently. “Greece would, through its laws, be able to amend contracts governed by Greek law or to be performed within the territory of Greece,” Mr. Clark said. “It is the governing law and the place of performance of the contract that is most important.” International contracts, which might be covered by English, German or Swiss law, would be more likely to be honored in the designated currency, though in some cases the wording of the legal document may be vague. And even if the law is on their side, companies would find that to extract payment from a Greek company, they would need a judge in Greece to enforce a ruling from a foreign court. “Enforcement of foreign judgments is harder or easier from country to country within the E.U.,” Mr. Clark said. “Greece has always had a reputation of being a difficult place in which to enforce judgments, from a practical perspective.” That means that international trading partners are likely to share in any losses that accompany a Greek exit from the euro. “International businesses that have long-term interests in Greece are going to have to be pragmatic and probably, in the short term, give some dispensation to their Greek counterparties, rather than trying to enforce the terms of contracts that cannot be performed,” Mr. Clark said.