Barclays said it will contact all affected blue rewards customers.
Barclays blue rewards customers are being refunded after it emerged that some perks had been underpaid.
The bank has said that it will contact affected customers, including those who have since left Barclays.
MoneySavingExpert com said some Barclays customers have received more than £100.
The bank said average payout is under £20 and it has added compensatory interest to apologise for the mistake.
Barclays told MoneySavingExpert that it uncovered the problem following one of its regular account reviews.
MoneySavingExpert.com said it was notified by someone who had opened a Barclays current account in 2017 to benefit from the perks of its blue rewards loyalty scheme.
He joined under a double rewards offer but challenged the bank after realising he was not being paid the correct amount, the website reported.
He said the bank failed to resolve the issue so he switched away.
He told MoneySavingExpert he was surprised to receive a letter four years later explaining he was due a £133 payment.
Helen Knapman, assistant news and investigations editor at MoneySavingExpert.com said: “If you believe you’re owed a payment but haven’t heard anything then get in touch with Barclays.
“If you don’t hear back or you’re unhappy with the response you do get, check if you can take your complaint to the free Financial Ombudsman Service.”
Barclays said it has already proactively contacted the majority of customers impacted, regardless of whether they still hold an account with it or not, and if anyone is due a refund, they will hear from it.
It insisted that those affected do not need to do anything.
Barclays said: “We are writing to a small number of customers who are part of our Barclays blue rewards, or previously held an account, to let them know that we owe them some backdated rewards.
“Customers do not need to do anything.
“We will be in touch directly with anyone who is due a refund to put this right.”