Last year, the UK government asked Sir Stephen to review whether Pip was “fair and fit for the future”. His initial report is being published on Thursday ahead of the final recommendations due in the autumn.
The interim report, published on Thursday,, external suggests a sweeping overhaul of the assessment system is needed.
It also highlights the steep increase in the number of Pip recipients in recent years and the forecast rise in spending on the benefit.
Pip is a benefit people with long-term illnesses and disabilities can claim if they need help with extra costs associated with living, work and care. Eligibility is determined through an assessment.
Under the current system, claimants are scored on a zero to 12 scale by a health professional on everyday tasks such as washing, getting dressed and preparing food.
Autism awareness campaigner Cheryl Fyfield said the Pip assessment process needed to change.
“It can be really hard to even qualify for Pip, let alone go through the process if you do qualify every three years,” she said.
“I’m autistic, I’m going to be autistic my whole life, yet every three years I have to go through the gruelling process to be reassessed.”
Sir Stephen told the BBC Pip “does a very important job in helping people meet the additional costs of disability”.
But he said disabled people had told the review the assessment can be “demeaning” and “deter you from participating in society”.
“We’ve also found that it hasn’t kept pace with changing understanding of health and disability over the last 13 years since the benefit was introduced, so we do think quite fundamental change is needed,” Sir Stephen said.
As of April there were about four million claimants entitled to Pip in England and Wales.
The number of Pip recipients has risen considerably since the benefit was introduced in 2013, with the increase fuelled by claimants citing mental health conditions in recent years.