Britain's biggest lenders will exit the UK at the start of 2017, a move taking place amid concerns over the Brexit negotiations.

Smaller banks are moving before Christmas, according to Anthony Browne, the chief executive of the British Bankers' Association, whose op-ed "Brexit politicians are putting us on a fast track to financial jeopardy" ran Sunday in the Observer newspaper.

"Most international banks now have project teams working out which operations they need to move to ensure they can continue serving customers, the date by which this must happen and how best to do it," wrote Browne. "Their hands are quivering over the relocate button. Many smaller banks plan to start relocations before Christmas; bigger banks are expected to start in the first quarter of next year."

Britain voted to leave the European Union in June and British Prime Minister Theresa May announced in early October that the UK would be on course to exit by March 2019.

Browne says the EU's integrated financial market has created jobs, helped EU citizens get better returns for their savings and made it "easier and cheaper for French farmers, German manufacturers and Italian fashion designers to secure funding."

"But it is now at risk," Browne writes. "It is underpinned legally by the "passporting" system enshrined in EU legislation, which allows banks based in the UK to sell services to customers in Europe, and banks based in Europe to sell services to customers in the UK, and access the global financial center that is London. It also allows banks based in one EU country to set up branches in any other EU country without going through local regulators."

Brexit, or UK's exit, could involve the UK leaving the single market, which allows authorized UK banks to conduct business freely in the 27 other European Union member states. French President Francois Hollande recently said "there must be a price" for Britain for leaving the Union.

UK firms could lose as much as $49 billion, according to Bloomberg, and, "put 70,000 jobs at risk."

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