LONDON (Reuters) - Britain's competition watchdog has ordered Royal Bank of Scotland RBS.L and Santander SAN.MC to appoint auditors to check how they remind customers who have payment protection insurance (PPI), amid a mis-selling scandal over the policies.

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The two banks failed to adequately remind customers about their PPI, some of whom might be due compensation as a result of Britain’s biggest ever consumer banking controversy, in which more than 36 billion pounds ($44 billion) has been paid back to those affected.

Both previously breached an order by the Competition and Markets Authority (CMA) requiring banks to send annual reminders to customers about PPI, the CMA said.

Spokespeople for both banks said the lenders have put in place measures to prevent a repeat of the errors, and apologized to customers affected by these mistakes.

RBS failed to provide reminders to almost 11,000 of its customers for up to 6 years, the CMA said, while Santander sent out annual reminders containing incorrect information to over 3,400 of its mortgage PPI customers from 2012-2017.

RBS has set aside 5.3 billion pounds for PPI compensation so far while Santander expects to reimburse up 1.6 billion pounds worth, according to data compiled by think tank New City Agenda.

“It is unacceptable that some banks aren’t providing PPI reminders - or are sending inaccurate ones - 8 years after our order came into force,” Adam Land, a senior director at the CMA, said.

“The legally binding directions we’ve issued today will make sure that both RBS and Santander now play by the rules.”

The eight year-long PPI saga is set to close on Thursday after Britain’s Financial Conduct Authority set a deadline for consumers affected by the scandal to seek compensation from the banks.

Friday’s sanction by the CMA will not lead to any extension of the FCA’s deadline for bank customers to contact their lenders about possible claims, a source familiar with the FCA’s thinking told Reuters.

PPI policies were typically sold alongside a personal loan or mortgage to cover repayments if borrowers fell ill or lost jobs, but thousands of customers were sold unsuitable policies and would never have been able to make a claim.