Shares in CYBG fell sharply on Thursday after the banking group warned it would take a hit of up to £450m on payment protection insurance (PPI) costs.
CYBG said it had received eight months' worth of information requests in August, the final month leading up to the PPI deadline.
More than 340,000 requests were received over five weeks.
CYBG said it could not "accurately determine the final outcome" of how many would lead to a payout.
The group, which owns Clydesdale and Yorkshire banks, has provisionally estimated the PPI legacy cost at between £300m and £450m.
Shares in CYBG slumped early on Thursday, with the stock at one point trading as low as 108.3p - down 22.6% on the previous day.
PPI was designed to cover loan repayments if borrowers fell ill or lost their job, but many were sold to people who did not want or need the cover.
On Wednesday, RBS warned that its profits could fall by a third this year after a surge of last-minute claims for mis-sold payment protection insurance ahead of the 29 August deadline.
The Co-operative Bank has also warned about the potential hit from PPI payouts.
The bank said it had "received a substantially greater volume of inquiries and complaints than expected in the final days prior to the complaint deadline".