The group, which also owns the B and Virgin Money brands, said it received an “unprecedented volume” of PPI information requests during August, ahead of the Financial Conduct Authority (FCA) complaint deadline of 29 August 2019.

As a result, CYBG has increased its provisions for legacy PPI costs by between £300m and £450m.

A statement from the bank said that as at the end of June the group’s PPI complaint costs were tracking broadly in line with its provision assumptions, with complaints slightly elevated but at a lower uphold rate.

Complaints increased slightly in July, but the financial impact was not expected to be material at that stage.

However, in August the group received more than eight months’ worth of PPI information requests in one month with about 340,000 enquiries in five weeks, of which 120,000 were received in the final three days.

The group also received a sustained increase in complaints during the same period, with an average of 5,000 per week during the first four weeks of August and an additional 22,000 complaints submitted during the final three days.

CYBG said it can estimate the costs associated with PPI complaints received given historical uphold rates, average redress and cost-to-do assumptions.

However, a key factor is the conversion rate of information requests into upheld complaints and the bank can’t accurately determine the final PPI provision necessary until it has processed more of the outstanding requests.

CYBG has estimated a provisional cost impact based on historical assumptions and an information request complaint conversion rate of 5 per cent to 12 per cent based on experience and sample testing. These calculations led to an estimated additional PPI provision of between £300m and £450m

CYBG said it expects to be in a position to provide a more accurate estimate of the costs when its full year results are published on 28 November 2019.

CYBG is the second bank this week to up its provision for PPI costs. The Royal Bank of Scotland said a late surge of complaints could cost it an additional £900m.