Six charities say people with serious illnesses are being found "fit to work" under new sickness benefits tests - and are urging changes to the assessment.
The MS Society, Parkinson's UK and others have urged changes to make the test "fairer" for people with illnesses where symptoms vary over time.
The Work Capability Assessment, currently being used for first-time claimants, is being reviewed.
The government says tests show the majority of new claimants can work.
It comes as protesters start arriving in Westminster to take part in the Hardest Hit march against benefit changes and other government policies they believe will have a negative impact on people with disabilities.
People claiming Employment and Support Allowance (ESA) - which replaced incapacity benefit - for the first time now have to go through a new test, the WCA, which was introduced under the previous Labour government.
The test is also in the process of being rolled out to 2.6 million people who were already claiming disability benefits.
But it has proved controversial and tens of thousands of people have appealed against its findings - with many appeals being upheld.
The group of six charities, which also includes the National Aids Trust, Arthritis Care, the Forward-ME group and Crohn's and Colitis UK, has published a report for consideration as part of Professor Malcolm Harrington's independent review of the WCA.
They say people with illnesses like Parkinson's, HIV or arthritis are "wrongly being found fit to work" after taking part in the assessment when applying for ESA.
The charities recommended 12 changes to the current WCA, including amending wording of the test to ask whether claimants can complete "activities reliably, repeatedly and safely", "within a reasonable amount of time", and "without significant discomfort, breathlessness or fatigue".
It suggests all claimants be invited to comment on how their condition affects them and suggests the Department for Work and Pensions develop a more specific definition of "work" based on the Australian system.
The report also says the severity and frequency of symptoms should be taken into account by assessors.
The charities said the WCA did not allow for people with symptoms that were worse on some days than others and points out that employers' attitudes to such people "remain unclear".
"We would welcome more systematic research to determine whether, in reality, someone whose condition means intermittent and unpredictable working would be considered for employment in the real world, although they might be considered 'employable'," the report says.
'Fair and consistent'
Simon Gillespie, chief executive of the MS Society, said: "As charities, we have been inundated by concerns from people living with a long-term health condition who've wrongly been found fit to work.
"Many of them want to work, but may require extra support to do so. Ensuring that the assessment is fair and consistent is therefore a vital task."
He said the report was just a first step and the charities were working alongside Professor Harrington and the Department for Work and Pensions on the issue.
The Work Capability Assessment determines whether applicants are entitled to the highest rate of ESA - for those deemed unable to work due to sickness or disability - or are considered "fit for work", in which case they are put on Jobseeker's Allowance instead.
It can also place applicants into a "work related activity group", where they will be expected to take steps to prepare themselves for work.
Professor Harrington published a report on the WCA last November, which the government fully endorsed and says it is acting on all his recommendations. He is currently undertaking a second review on further issues around the assessments.
A spokesman for the Department of Work and Pensions said: "We want to ensure the Work Capability Assessment is as fair and accurate as possible and have already accepted all of Professor Harrington's recommendations from the first year of his independent review.
"In year two he will specifically look at fluctuating conditions and we look forward to receiving his recommendations later in the year".
Last month the government released figures for assessments of people claiming ESA for the first time between October 2008 and August 2010.
The figures showed 887,300 of 1,175,700 people applying for employment and support allowance (ESA) between October 2008 and August 2010 failed to qualify - either because they were found "fit to work" or dropped their claim before it was completed.
Only 6% of claims - 73,500 people - were considered to be entitled to full ESA support.
Employment minister Chris Grayling said at the time many benefit claimants had been "simply abandoned on benefits" and would be reassessed and given specialist back-to-work support.
He said those who could not work would get "unconditional support" but for others it was "right and proper that they start back on the road to employment".