A shake up of benefits payments for sick and disabled people could create a two-tier system, MPs have warned.
Labour MPs have rebelled on previous plans to reform the benefit
The Commons work and pensions select committee said it was concerned government plans could leave some claimants worse off.
The MPs have been examining proposals to replace incapacity benefits, paid to 2.7m people, with a new employment and support allowance.
A Department of Work and Pensions spokeswoman said reform was vital.
The government is trying to get a million disabled people off benefits and into jobs, saying it will save £7bn a year.
The spokeswoman said: "It is vital that we continue to build on our reforms and ensure we have a welfare state fit for the 21st Century.
"Our welfare reform bill, which will be published later this year, will be key to achieving that."
Under the plans more severely disabled people would receive a higher rate of benefit and have no obligation to look for work.
But it would be compulsory for claimants assessed as able to work to take part in schemes designed to help them back into jobs, including counselling, training and advice.
Those who refused to take part in back-to-work schemes would risk losing part of their benefits.
Opponents - including some backbench Labour MPs - say they are concerned that some of the most vulnerable in society are being targeted to save the government money, and that there will be undue pressure placed on them to come off benefits.
The select committee report to be published on Saturday will say ministers must ensure the reforms would not make disabled people on the lower rate worse off.
The MPs will also demand urgent clarification on how the government's target, of getting one million disabled people off benefits and into jobs, is to be calculated.
The report also questions whether the government has allocated enough money to provide specialist teams in job centres to help disabled people into jobs.
BBC social affairs correspondent Sophie Hutchinson said there was also concern in the select committee that the plans failed to tackle employers' often negative attitudes towards disabled people.
"They insist much more must be done to encourage employers to open their doors to those with disabilities," she said.