Swiss financial institutions could lose the half a billion Swiss francs it had passed onto Britain's tax authorities.

According to the Swiss Bankers Association (SBA), the financial institutions are calculating potential losses following the British-Swiss deal aimed at sweeping Swiss banks clean of undeclared money held in accounts of UK clients.

As part of the agreement, which came into force earlier this year, UK citizens' cash held in Swiss bank accounts would now be taxed at 21% and future capital gains at 41%, in return for the preservation of their anonymity.

Swiss banks have already made an initial payment of 500m Swiss francs (€405m, £347m, $522m) to the UK government earlier this year.

Once British authorities have received 800m Swiss francs in total, then the banks can start recovering this amount from the UK's HMRC.

Switzerland has also agreed a similar withholding tax deal with Austria, which also goes into effect this year but a similar third deal was rejected by the German upper house of parliament.

However, the SBA said that some incumbent Swiss banks have found that there were fewer untaxed UK assets in the Alpine country than previously estimated. It added that many British clients are not liable for UK tax because they are resident in Switzerland.

Credit Suisse said in a statement that is expecting to be hit by a 90m Swiss francs loss following the British-Swiss deal and said the after-tax impact would be included in its second quarter 2013 results.

UBS told IBTimes UK that "our share of the upfront payments under the Swiss-UK tax agreement is approximately 100m Swiss francs."

"We will take into account all information available to us in our assessment of the impact of the arrangement on our second quarter results, which will be reported on 30 July 2013."

In January this year, the UK tax authority revealed a sharp increase in potentially underpaid revenue from some of the country's largest businesses, after identifying a number of tax avoidance methods that companies use in order to pay less.

As of 31 July 2012, the amount of 'tax under consideration' from large businesses, in relation to these tax avoidance methods including transfer pricing and reporting thin capitalisation, stood at £1bn compared with £680m a year earlier.