Royal Bank of Scotland could be facing as much as £5 billion in legal claims from shareholders over its 2008 rights issue, as groups seeking damages from the bank report a late surge of interest from investors ahead of a May 15 deadline.

According to people close to the shareholder action groups, investors have added claims worth hundreds of millions of pounds in the past few weeks. There is considerable uncertainty over the final totals, however, until formal claim documents are filed.