RBS plans to "proactively" refund overdraft fees if it loses the ongoing test case over bank charges.

The bank, which also owns NatWest, confirmed today that it was making “careful contingency plans” after a document referring to the measures was leaked to the BBC.

According to the document, a team is "preparing systems and processes to proactively refund charges to the group's customer base" and "all customer accounts that are due a refund will be calculated as accurately as possible".

The bank is one of eight lenders fighting a High Court test case with the Office of Fair Trading (OFT) to decide the legality of unauthorised overdraft fees, which cost consumers billions of pounds a year. The banks appealed after losing the first round in April, when the judge ruled that the OFT could apply consumer contract regulations to decide if the charges are fair or not.

The case – which the banks could pursue to the House of Lords – has left up to one million savers who are pursuing their banks for refunds in limbo, after the Financial Services Authority (FSA) authorised an extendable waiver that allows banks not to pay out on claims until next January. RBS said today that the conditions of the waiver were behind the contingency plans leaked today.

A spokesman for RBS said: "As a condition of the waiver, RBS and the other banks agreed with the FSA in July 2007 and again in 2008, to deal 'efficiently and swiftly' with customer complaints on conclusion of the test case, irrespective of its outcome. This included 'making preparations for dealing with relevant charges complaints when this direction ends'.

"For an organisation of our size and our different brands, complying with these requirements demands careful contingency planning and this document merely confirms that RBS is taking its obligations in this respect seriously as it has done throughout the whole test case process. This workstream has absolutely no bearing on how we see the outcome of the test case."

Campaigners have criticised the banks for prolonging the legal process. Phil Jones, of Which?, the campaign group, said: "The banks have already lost the test case in the court of public opinion. Instead of stringing out the legal action, perhaps even to the House of Lords, the banks should admit defeat and repay their customers what they have unfairly charged. Only by doing this can they start to rebuild the trust they have lost amongst the public."

An verdict on the banks' appeal is expected before the end of the year. However, final resolution of the case is unlikely before the end of next year. As long as the waiver is in place, consumers can claim for refunds on charges dating back to July 2001. The straightforward process is outlined here.