Disability Wales fears that delays in awarding Personal Independence Payments (PIP) are causing hardship.
There are reports of some claimants waiting up to 11 months for their first payment.
The Department for Work and Pensions (DWP) said it is committed to driving down waiting times.
PIP began to be rolled out across the UK in 2013 and will eventually replace Disability Living Allowance (DLA) for working-age claimants.
It was introduced on the grounds that DLA, where the majority of claimants are automatically granted the benefit for life, was “outdated and unfair” to claimants and taxpayers.
Unlike DLA, PIP involves a face-to-face assessment by a health professional for the majority of claimants, together with regular reviews.
The most recent government statistics show that of the nearly 530,000 PIP applications made since the benefit was launched, around 320,000 – about 60 per cent – were still to be decided.
Within those figures there are examples of people waiting 10 or 11 months before their claim is approved.
Gemma Leigh, an advisor with the cancer charity Tenovus, has one client who recently received his first payment after applying for PIP in November 2013.
“Financially, the family were really struggling. He will have been awarded about £130 a week now.
“But that would have been money that he should have been receiving all along because the family need it.”
In a statement, the DWP told BBC Radio Wales’ Eye on Wales programme that by the end of 2014, no-one will have to wait longer than 16 weeks for an assessment.
Paul Swann of Disability Wales said: “We support the call that the process should be called to a halt until timescales, from a claimant’s perspective, are brought down to a reasonable level.
“Quite frankly a 16-week wait is still totally unacceptable.”
In Wales, assessments have been contracted out to private firm Capita.
Its director of disability assessment, Stephen Duckworth, admits that it was overwhelmed in the early days of the benefit but says it is now on track to meet the UK Government’s targets.
Eye on Wales is broadcast on BBC Radio Wales at 12:30 GMT on Sunday 26 October.