ZURICH, Dec 21 (Reuters) - Swiss banks have agreed how to contribute to a 2.5 billion Swiss francs ($2.7 billion) down payment to Britain and Germany as part of tax deals reached in August over undeclared assets held in hidden accounts.
Banks will make a payment based on a formula combining the size of managed assets and the tax withheld, the Swiss Bankers Association said on Wednesday.
The 30 banks with the most managed assets from British and German clients at the end of 2010 will pay Germany 2 billion francs and Britain 500 million upfront, which they hope to recoup when they start collecting taxes from clients.
UBS and Credit Suisse, with 273 billion francs and 253 billion, respectively, in total European assets, are likely to be responsible for most of the payment. Neither strip out how much they manage in Britain or Germany alone.
The lobby declined to comment on a recent European Union threat to challenge the deals because they may infringe EU law.
Switzerland sees the British and German agreements as a blueprint for others in the EU. Greece said last week it was nearing formal talks with Switzerland over a similar deal.
While secrecy has helped Switzerland build a $2 trillion offshore finance centre, it has faced intense international pressure on tax evasion in recent years, as cash-strapped governments seek to boost revenues. ($1 = 0.9294 Swiss francs) (Editing by Dan Lalor)