Financial Conduct Authority (FCA) figures suggest there has already been a recent surge in activity by consumers - and it is now gearing up for a last-minute flurry of inquiries.
An estimated 64 million PPI policies were sold in the UK - many in the 1990s and early 2000s.
The deadline for making a claim is 11.59pm on August 29.
The FCA's figures show firms have paid back £36bn since January 2011 to customers who complained - including £340.4m in June.
But nearly a fifth (18 per cent) of 2,000 consumers surveyed for the FCA say they will make a last-minute decision on whether to complain about PPI. The regulator is urging them to act now.
Just over half (53 percent) of these "last minuters" are confident they will complain ahead of the deadline.
Martin Lewis, founder of MoneySavingExpert.com said: "While PPI reclaiming is easy to do, and can be done for free, it is easier if you've got the documents and it can take a while to search through your old files - first to find out who your lenders were, and then to find out if you had PPI in the first place.
"If you don't know, help is still available, but all of these things take time."
In the past eight weeks since the FCA's final PPI push went live, it has seen a 420 per cent surge in web users, and a 269 per cent increase in calls, compared with the previous eight weeks.
More than 5.1 million users have now accessed the FCA's PPI website which offers help with claiming and 84,733 calls have been made to its dedicated contact centre.
PPI (payment protection insurance) was routinely added to products such as store cards, credit cards or mortgages.
It was intended to protect people if they could not keep up with their payments, due to illness or unemployment for example. But it was widely-mis-sold.
People were pressured into buying it, did not know they had it, or it was unsuitable.
It is the most complained-about financial product that the Financial Ombudsman Service (FOS) has ever seen. It has received over two million complaints.
Crucially, even if people do not think they were mis-sold, there is also now another reason to complain about PPI.
Following a court case known as "Plevin", people can also complain on the basis that a high level of commission was earned from the sale of the policy - but they were not told about it.
People can complain about commission even if they have had a previous complaint about mis-selling of PPI rejected.
The FCA's website includes a list of providers that have sold PPI at www.fca.org.uk/ppi/how-to-complain/search-for-provider.
This includes high street stores, catalogue firms, building societies and supermarkets.
There is also plenty of free help available for making a PPI claim on websites including MoneySavingExpert.com, Resolver.co.uk and the Which? website.
People making complaints to financial firms must give them time to deal with their gripe.
But if they are not happy with the outcome they can then go to the FOS - provided the initial complaint they made to the financial firm that sold PPI was within the August 29 deadline.
Emma Stranack, the FCA's PPI deadline campaign lead, said: "Leaving things to the last minute is natural behaviour for many of us and we're sharing these findings to show that anyone who hasn't yet decided on PPI is not alone in leaving things close to a deadline.
"That said, now is the time to act. We all lead busy lives, and the FCA is prepared for a last-minute flurry of inquiries.
"We've extended our PPI helpline hours to 8pm on weeknights and 5pm on Saturdays to provide further support to consumers and will be available for calls on Bank Holiday Monday.
"Ultimately, we don't want the UK public to miss their chance to decide.
"So, if you think you might have had credit with a PPI policy attached - particularly in the 90s or 00s - now is the time to get in touch with your provider.
"You need to submit your claim by 29 August, or you won't be able to claim money back for PPI."
- FCA support is available online at fca.org.uk/ppi or by calling the FCA helpline on 0800 101 8800.