The competition watchdog has told Royal Bank of Scotland and Santander to fix their PPI processes, after finding failings by both banks in their communications with customers.
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According to the Competition and Markets Authority (CMA), RBS failed to provide annual PPI reminders to almost 11,000 customers for up to six years.
Santander, meanwhile, sent out annual reminders which contained incorrect information to more than 3,400 of its mortgage PPI customers from 2012 to 2017.
The failures contravene an order made in 2011 following the Competition Commission's investigation into PPI, which required providers to send customers an annual reminder setting out how much was paid for their policy, the type of cover they had, and their right to cancel.
Both have been instructed to appoint an independent auditor to assess their PPI processes.
It is not the first time both RBS and Santander have been in hot water over the order. In 2016 both banks were warned by the CMA to improve their PPI practices.
Adam Land, the CMA's senior director of remedies, business and financial analysis, said: "It is unacceptable that some banks aren't providing PPI reminders - or are sending inaccurate ones - eight years after our order came into force. The legally binding directions we've issued today will make sure that both RBS and Santander now play by the rules.
"These are serious issues that, in the future, may result in fines if the Government gives us the powers we've asked for.
"For now, we expect RBS to repay all affected customers quickly, and for both RBS and Santander to make sure that similar breaches do not happen again."
It comes with just one week left for customers to make a complaint about PPI to their provider, or risk missing out on the chance of a refund.
The deadline for making a claim is 11.59pm on August 29.