The British bank admitted in a statement that it had told a “very small percentage of customers” who contacted it via claims management firms that they did not hold PPI policies when they did.
Barclays said it would be "proactively contacting" affected customers to rectify the situation.
PPI offered cover for anyone who could not keep up loan repayments if they fell ill or lost their jobs, but it was often sold to people for whom it was unsuitable. In many cases, customers did not even realise they had opted to take out PPI.
Barclays said that in "less than 1.1%" of those cases where requests came via claims management companies, customers were wrongly told that they did not have PPI.
Barclays said the error occurred when bulk requests made by claims management firms on behalf of individuals were not always checked against some of the records they hold.
It did not say exactly how many customers were affected, but the BBC reported that the figure was likely to be in the tens of thousands.
The bank last month said its first-half profits were partially hit by a £400mln provision for PPI claims, although this was lower than last year’s £700mln charge.
Shares in Barclays were 0.32% up at 175.68p in early afternoon trade.