Barclays' payment protection insurance (PPI) bill has climbed to £4.85 billion, after it announced it has set aside a further £900 million to pay for its part in the mis-selling scandal.

The bank says its decision came after a "lower than expected decline in claims" (see our PPI Reclaiming guide to find out how to do it yourself for free).

According to the Financial Conduct Authority, £15.1 billion was paid out between January 2011 and May 2014 by the firms that make up the vast majority of PPI complaints.

But as this figure doesn't take into account cash set aside by banks that haven't yet been paid out, or payouts made over the past few months, the overall PPI bill will end up being much larger.

Consumer group Which? estimates the big five banking groups' – Barclays, HSBC, Lloyds, RBS and Santander – total cost so far is £23 billion.

'Staggering' sum

Guy Anker,'s managing editor, says: "As if the mis-selling scandal wasn't big enough this latest, huge increase in provision takes the big five's total to a reported £23 billion.

"It's a staggering amount of money and shows the enormous nature of the systemic mis-selling scandal that has polluted the banking industry.

"It's vital anyone who thinks they've been mis-sold PPI gets in a claim to get their money back.

"And while you're at it, don't fall into the trap of paying a typical 30% of your payout to a claims company when you can do it easily, and for free, yourself."

The mis-selling figures were released as Barclays today revealed a 7% slide in first-half of the year profits. The bank said its adjusted pre-tax profits for the first six months of this year were £3.35 billion.