However, many United States banks are unfazed by the Norway model in which the UK would remain in the European Economic Area free-trade zone, because they already essentially see themselves as rule-takers, the people said.
"One of May's Conservative predecessors, John Major, last week described Brexit as the "worst foreign policy decision since the Second World War" and dismissed suggestions the fractious divorce from Brussels could herald a return of the country's wartime 'Blitz spirit", which would help Britain overcome the wrenching dislocation of crashing out.
After the British parliament rejected the Brexit withdrawal agreement in January, German carmakers urged the British government not to leave the bloc without a trade deal.
In 2017, Switzerland registered 11.4 billion Swiss francs (US$11.4 billion) in exports to Britain, making it the wealthy Alpine nation's sixth largest export market.
Britain has long traded around the world under the terms of deals agreed by the European Union, a procedure that will come to an end after Brexit.
It is also meant to postpone a revolt by cabinet ministers who want to take a no-deal Brexit off the table for good.
"One of the things I would like our teams to discuss is the exact nature of those alternative arrangements", May wrote.More news: Venezuela open to barter trade with India to boost oil sales
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Boris Johnson accused Mr Corbyn of trying to "frustrate" Brexit by demanding a customs union, which he said would prevent the United Kingdom from striking its own trade deals.
Britain and Switzerland signed a deal on Monday to preserve trade relations between the two countries even if London leaves the European Union without a deal with Brussels.
Switzerland meanwhile is Britain's fifth biggest export partner outside the EU.
The accord follows a Swiss "Mind the Gap" strategy developed after British voters approved Brexit. It covers issues like free trade, public procurement, agriculture and the fight against fraud.
She is unlikely to signal any shift towards a closer future relationship with the EU, after writing to Jeremy Corbyn to underline her continued objections to a customs union, and instead she will focus on the backstop. He says May's agreement can achieve "a great deal of what Jeremy Corbyn is interested in without taking away that option of having other trade deals".
But the Irish government and the European Union have repeatedly rejected calls for changes.
The PM had been expected to deliver her statement on Wednesday ahead of a Commons debate on Thursday, but her official spokesman said the earlier timing would "give Parliament a couple of days to digest the content".
A "hard" departure without a deal would see tariffs imposed at the border, "tangling up global supply chains", study co-author Oliver Holtemoeller said in a statement.