The number of economically inactive UK-born adults has risen by 833,000 since Covid, with an increase of 230,000 among those born overseas.
Ms Reeves has vowed to make “difficult decisions” on welfare in her maiden Budget on Oct 30.
She said on Tuesday: “There is more to do in supporting people into employment because if you can work, you should work.”
DLA is a non-means-tested benefit paid mainly to parents of children who need help with extra caring costs.
Payments can rise to a maximum of just over £9,500 a year for children with the most severe conditions.
More than half of children with the most common mental health conditions receive just under £3,800 per year.
This accounts for those who need frequent care or continual supervision during the day, who are also granted an extra £1,500 as part of a mobility allowance.
Parents of children in Scotland can also claim higher rates of other benefits if their child is classified as disabled.
It is unclear whether Labour will proceed with the Conservatives’ benefits overhaul that would have narrowed eligibility for incapacity benefits, saving £3bn a year by the end of the decade.
It is also uncertain whether the government will take forward proposals to reform disability benefits for people of working age, known as Personal Independence Payments (PIP).
There are currently a record 3.5m people receiving PIP, with 30,000 new claims submitted each month.
This is up from around 17,000 pre-Covid, costing the Treasury an extra £15bn each year by the end of the decade.
