Royal Bank of Scotland has swung to a quarterly loss after being forced to put aside an extra £900m to cover a surge in payment protection insurance complaints before the claims deadline.

The extra charge means RBS slid to an operating loss of £8m for the three months to September, compared with a profit of £961m in the same quarter last year.

PPI has become by far the banking industry’s biggest mis-selling scandal and this latest charge for RBS is at the top end of its estimates in September, after the August claims deadline.

The bank had forecast a third-quarter provision of between £600m to £900m.

The RBS finance director said it was too soon to say this was the end of the PPI saga, given the volume of claims the lender was still working through. “I think it would be very brave … to say the line was completely drawn under it but this is certainly our best estimate,” Katie Murray said.

RBS, which is still 62%-owned by the government, said its PPI provisions total was £6.2bn to date. The bill for the industry as a whole has already exceeded £42bn.

Despite the third-quarter losses, RBS said it remained on track for full-year expectations in uncertain times.

The economic outlook forced RBS to take a £55m charge, but Murray said the “slight strain” was due to more than just Brexit jitters. She said it reflected deteriorating global growth forecasts in recent months, as well as ongoing volatility.

“The economic indicators have got that much worse since the end of June,” she said. “It really is sort of the growing volatility in the economics that we see rather than something politically tied towards Brexit, I think it’s important to have the separation.”

The lender said it was on track to cut costs by £300m by the end of the year, but Murray refused to speculate on the future of RBS’ investment bank, NatWest Markets, which suffered a £193m loss for the quarter.

“As for job cuts, you know we always prefer to talk to our own staff, before we spoke externally.”

RBS said it cut 900 jobs, approximately 6% of its total headcount, in the third quarter to about 65,700 staff.

The figures are the last for the chief executive, Ross McEwan, who hands over to Alison Rose next week. Rose, who has been with the bank for 27 years, will become the first woman to head a British high street bank.

She is the deputy chief executive of NatWest Holdings, RBS’s retail and commercial banking division, and also runs RBS’s commercial and private banking business, including Coutts.

Rose is expected to deliver a new strategic plan for the entire bank in February.