Around 284,000 are being urged to contact the department for advice after the Supreme Court ruled last week that thousands of sick and disabled people have been underpaid.
Recipients of PIP generally have a long-term physical, mental health condition or disability - or difficulty doing certain everyday tasks such as getting around because of a condition.
The government may now be forced to pay more than £74 million worth of benefits. It all came about after a review found the definition of people eligible should be widened.
This is what it all means.
How many backdated payments have been made?
Around 14,000 arrears payments, amassing some £74 million, have been made.
Disabilities minister Tom Pursglove said: "Given the complexity of the exercise we started at a relatively small scale, prioritising terminally ill and recently deceased claimants, testing our processes and communications with claimants, to ensure they are effective before ramping up.
"We are monitoring the numbers of, and reasons for, revised awards closely and making regular quality checks to ensure our decision-making is accurate and fair.
"Confident that reviews are achieving the right outcomes for claimants, we have completed upskilling additional staff available for this exercise and expect to complete the review of all cases available to the exercise by the end of 2025."
How to claim back payments
Start by calling the PIP enquiry phone line on 0800 121 4433 from 9am to 5pm Monday to Friday. For further help, contact Citizens Advice on 03444 111 444.
How long does it take for DWP to pay backdated money?
PIP is paid directly into your account and will begin soon after you get your decision letter.
It is then usually paid every four weeks.
The DWP has not said when or how backdated payments could come.
Who is eligible?
You can get Personal Independence Payment (PIP) if all of the following apply to you:
You’re 16 or over,
You have a long-term physical or mental health condition or disability,
You have difficulty doing certain everyday tasks or getting around,
You expect the difficulties to last for at least 12 months from when they started,
You must also be under State Pension age if you’ve not received PIP before.
Who is not eligible?
The Department for Work and Pensions have said they will not review a claim if:
The enhanced rate of the daily living part of PIP has been awarded continuously since 6 April 2016,
A tribunal made a decision on a claim since 6 April 2016,
A decision not to award PIP was made before 6 April 2016.