The firm carrying out fitness-for-work assessments for the government lacks disabled access at a quarter of its premises, MPs have heard.
Employment minister Mark Hoban said 31 of 123 centres used by Atos lacked ground-floor access for wheelchairs.
Six centres in particular had "terrible" problems, causing almost three-quarters of case backlogs by failing to inform applicants.
But Atos said the majority of sites it used were government buidings.
The government is using the French firm to carry out face-to-face assessments of disability benefit claimants' eligibility - known as the "limited capability for work assessment".
Those judged able to do some work are given support to find jobs, while those judged able to work full time are moved on to Jobseekers Allowance.
The Commons Work and Pensions Committee heard evidence from Mr Hoban, who said: "There's a challenge in ensuring interview centres are accessible. What we don't want to do is to get people to turn up to centres that they can't effectively access."
Mr Hoban said that, in cases where wheelchair access was not possible, home visits were offered as an alternative.
He revealed that 31 of Atos's 123 sites did not have such access. Six centres - in Croydon, Ealing, Birmingham, Luton, Mansfield and Norwich - were responsible for 73% of the case backlog.
Mr Hoban said: "I think it's terrible. I think it's unacceptable that six centres account for 73%."
He added: "I think it's something that, over time, we should drive out."
The committee's chairwoman, Labour MP Dame Anne Begg, said it would be difficult for the Department for Work and Pensions to ask businesses to "put in access when the department cannot itself" guarantee it in buildings used for assessments.
Lib Dem MP Stephen Lloyd described the situation as "potty".
A spokeswoman for Atos healthcare, speaking after the hearing, said all assessment centres met "accessibility standards".
"We let people know about access prior to scheduling appointments to try to ensure no one goes to a centre that isn't appropriate for them. We lease only 20 of the 123 centres we use, the remainder are government buildings.
"We are working with the Department for Work and Pensions to ensure there is appropriate access at all sites used and hope that this will be the case shortly."