The report also raised concerns about shortening the first meeting Universal Credit claimants have with a work coach from 50 to 30 minutes.
It warned that without mitigating action from government, "claimants with more complex needs may not get the support they need".
Last year, the government abandoned plans which would have made it harder for people to claim Pip, in the face of a major rebellion by Labour MPs.
The government had estimated the proposals would have saved £5bn a year by 2030.
Instead it launched a review into Pip, led by Minister for Social Security and Disability Sir Stephen Timms, which is expected to report back by the autumn.
The government has said the purpose of the the review is to ensure Pip is "fair and fit for the future rather than to generate proposals for further savings".
A DWP spokesperson said: "We're fixing the broken welfare system we inherited by giving claimants the support they need to move into good, secure jobs and out of poverty.
"We've redeployed around 1,000 work coaches to help sick or disabled people who have been left behind, alongside the most ambitious employment reforms for a generation.
"These reforms are being delivered as we replace outdated systems through our ambitious £647m modernisation programme."