The number of complaints about the mis-selling of PPI insurance has risen by 40% in just six months, according to the Financial Conduct Authority (FCA).
Between the first and second half of 2017, the number of complaints rose to 1.55 million, the highest figure in four years.
The period coincided with the start of an FCA advertising campaign, involving Arnold Schwarzenegger.
In a series of adverts, he advises people to "do it now."
All complaints about PPI mis-selling have to be filed by 29 August 2019.
- Arnie returns to encourage PPI complaints
"Having set a deadline for PPI complaints, we are encouraging consumers to decide whether they want to claim, and if they do, to make their complaint as soon as possible, as many already have," said Christopher Woolard, executive director of strategy and competition at the FCA.
"We are continuing to monitor and challenge all firms to ensure they maintain the expected standards, and are delivering on their commitments to make it easy for people to complain about PPI."
The FCA said the total amount paid out for PPI mis-selling has now reached £30bn. That's around twice the cost of London's Crossrail project.
Payment Protection Insurance was sold with credit cards, loans and car finance contracts.
It was designed to protect people if they became ill or unemployed, but in many cases the insurance was unnecessary or inappropriate.