LONDON (Reuters) - Royal Bank of Scotland is planning to "pro-actively" refund overdraft fees to customers if banks lose a court case over bank charges that could cost the industry billions of pounds.

RBS, which owns NatWest and has 13 million UK customers, has a team "preparing systems and processes to pro-actively refund charges to the group's customer base", according to an internal document seen and reported by the BBC.

RBS said on Wednesday it had agreed to deal swiftly with outstanding complaints once a test case ends, and the leaked document showed the contingency planning needed at a bank of its size.

"This planning has absolutely no bearing on how we see the outcome of the test case," a spokeswoman for the bank said.

All top banks stand to pay out millions of pounds in refunds to customers if they lose the test case, which began early this year in the High Court and is expected to last for many more months.

HSBC (HSBA.L: Quote, Profile, Research) said in March it could take a $600 million (406 million pound) hit if it loses the court case. No other bank has estimated the likely cost of defeat, but analysts have estimated it would cost the industry over 2 billion pounds in lost revenue and refunds.

Seven banks and a building society and Britain's consumer watchdog want the High Court to clarify a long running dispute on the charges applied to unauthorised account overdrafts.

Thousands of customers have reclaimed the charges, typically between 24 and 39 pounds for each unauthorised overdraft, after a high profile customer backlash last year. Banks paid out more than 400 million pounds in refunds in the first half of 2007, but say the charges are fair and clear.

(Reporting by Steve Slater)