Barclays reported a 33% drop in half-year profits to £2.6 billion after it took a £1 billion hit to cover compensation for customers who were mis-sold payment protection insurance.
The hit comes after the High Court decided that new rules on the mis-selling of PPI could be applied retrospectively and the British Bankers' Association pulled an appeal against the decision.
It means banks must contact those they believe may have been mis-sold in the past.
Lloyds Banking Group made a provision of £3.2 billion, Royal Bank of Scotland set aside £850 million and HSBC earmarked £270 million.
Only last week, Santander revealed PPI mis-selling had cost it £540 million, taking its profits after tax and the PPI charge to £300 million (see the Santander £540 million news story).
However, Barclays's underlying performance stripping out the PPI provision revealed profit growth. Without it, Barclays would have seen profits increase 24% to £3.7 billion in the period.