Government plans to overhaul incapacity benefit are unrealistic, Northern Ireland Law Centre director Les Allamby has said.
The timetable for shifting claimants to a new allowance would mean dealing with around 500 people a week for three years, the association of solicitors added.
The Department for Work and Pensions estimates that almost a quarter of existing claimants will fail the new test for work capability.
Mr Allamby said: "The migration of incapacity benefit claimants to employment and support allowance has set an ambitious or unrealistic timetable.
"In addition, with the tightening of the work capacity assessment test the expectation is that over two thirds of new claimants to ESA will also fail the test.
"Whether the Social Security Agency and appeals service have the capacity to deal with the likely numbers appealing remains to be seen.
"Social development minister Alex Attwood has pledged to resist the Coalition's 10% cut in housing benefit for people on job seekers allowance for over a year.
The changes were announced in October. Northern Ireland welfare is funded directly from Westminster.Any deviation from the current system may leave the executive "picking up the tab" for Northern Ireland's entire £3 billion social security bill, according to DUP MLA Simon Hamilton.
The reductions are part of a planned £18 billion reduction in welfare payments detailed in the chancellor's comprehensive spending review.