RBS has run up legal costs of £86.2m, with the total rising to £103.4m once 20% VAT is added.

Since the lender is 70% state-owned, taxpayers will have to foot most of the bill.

The case has been filed against the bank and four of former directors, including ex-chief executive Fred Goodwin.

RBS has denied wrongdoing and was reported to have offered £700m to settle the case in July, but the offer was rejected by claimants.

The case is due for trial in March next year and could last for six months.

Earlier this week, RBS reached a $1.1 billion (£846m) final settlement with a US regulator over claims that it mis-sold mortgage securities in the run-up to the financial crisis.