Ministers were urged today to ensure eligibility tests for a new disability benefit are "accurate and fair" before they are rolled out nationally.
The Work and Pensions Select Committee said there were "high levels of anxiety" among claimants of Disability Living Allowance (DLA), which is to be replaced with the new Personal Independence Payment for those of working age.
The Treasury has projected 20% savings under the new system, to be introduced in 2013, but it was still unclear which claimants stand to lose their benefits, the committee said in a report today.
Committee chairwoman and Labour MP Dame Anne Begg said the new assessment must avoid the mistakes of that used for Employment and Support Allowance (ESA) from 2008, which had a "mechanistic, box-ticking approach"
"The Government's own estimates show that 500,000 fewer people will receive support by 2015-16 compared to the situation if DLA for working-age claimants had continued," she said.
"Announcing the change against a background of budget cuts, and the previous negative experience which many people have had with the Work Capability Assessment (WCA), has created high levels of anxiety amongst DLA recipients."
The committee said the tests should not be implemented nationally until a limited regional trial had proved successful.
"The Government needs to be certain that the new assessment procedure is accurate and fair before it is introduced," Dame Anne said.
"Otherwise, there is a risk that people with serious disabilities and health conditions will lose the money they rely on to meet the additional costs incurred as a result of their disability - costs such as maintaining wheelchairs, using specially adapted cars, or paying for help to ensure they can live independently."
Disability campaigners urged the Government to heed the MPs' report.
Richard Hawkes, chief executive of disability charity Scope said: "The Government wants to create a more targeted payment. But if it goes ahead with the proposed assessment there is no guarantee that support will go to the people that need it.
"Day-to-day life costs more if you're disabled. PIP should be a financial lifeline for disabled people, but if they get this wrong many more will be being pushed further into poverty."
Simon Shaw, parliamentary manager at the deafblind charity Sense, said: "We urge the Government to change the way it assesses people while there is still time.
"We specifically welcome the Committee's recommendation that the Government consider the effectiveness of face-to-face PIP assessments for people with impairments that are severe or unlikely to change, such as deafblind people, when there is sufficient written evidence from medical and social care professionals with knowledge of the person's disability."
Steve Winyard, head of policy and campaigns at RNIB, said the Government had "a lot of work to do" before the PIP - contained in the controversial Welfare Reform Bill which has been repeatedly defeated in the Lords - was introduced.
"Blind, partially sighted and other disabled people must not find themselves forced to go through lengthy, costly and stressful appeals processes to get a fair outcome," he said.
The Department for Work and Pensions said DLA was "outdated".
"Under the current system there are hundreds of millions of pounds of overpayments and underpayments and 70% of people get the benefit for life without systematic checks to see if their condition has changed," a spokeswoman said.
"We are replacing DLA with Personal Independence Payment and introducing a new face-to-face assessment and regular reviews to make sure people are getting the right levels of support.
"We have been working with disabled people and around 60 disability organisations from the start to get the assessment right, and have made significant changes as a direct result of the comments we have received - improving the assessment and making it fairer."