Hundreds of thousands of people have been affected by a benefits backlog, the government has said.
Minister for Disabled People Mike Penning told a committee of MPs that more than 700,000 people were waiting for assessments.
He said the assessments were for employment and support allowance (ESA).
Prime Minister David Cameron told MPs that benefits changes should be carried out “in a way that works well” rather than to an “artificial deadline”.
He was responding to a question in Parliament by Labour MP Katy Clark, who asked why the majority of those who had applied for the personal independence payment (PIP) had yet to receive a decision.
PIP started to replace the disability living allowance from April 2013.
Mr Penning blamed the ESA delays on failings with Atos, the contractor responsible for carrying out controversial fit-for-work tests.
An official from the Department for Work and Pensions (DWP) told MPs the firm “couldn’t deliver the quality at the capacity we want”.
The DWP said the total backlog stood at 712,000 people. Of these, 394,000 are new claimants for ESA and 234,000 are existing ESA recipients whose reassessments as to whether they are still entitled to the benefit have been delayed.
A further 84,000 are people still on incapacity benefit who have not yet been moved over to ESA.
Mr Penning said the government had failed to meet its own deadline of moving these people onto the new benefit by April.
They are still awaiting assessment.
Critics have said delays and wrong decisions in Atos’s work have caused distress to vulnerable people.
Atos has acknowledged difficulties but said its staff are well trained and it has become a “lightning rod” for public anger with the principle of fitness-to-work assessments.
It said its staff had been “vilified” and subjected to intimidation by some claimants merely for doing what was asked of them by ministers.
Timely and proper
In total, more than three million people on ESA, including all those who previously claimed incapacity benefit, are being assessed to see how their illness or disability affects their ability to work.
This process began under the last Labour government and has been accelerated by the coalition government.
Mr Penning said a decision in March to end Atos’s contract early was based on the fact he had lost faith in the firm.
He said a new contractor – expected to be appointed in early 2015 – would not be chosen on cost alone but on its ability to carry out the assessments in a timely and proper fashion.
This would cost the department more money, he said.