Hundreds of thousands of severely ill and disabled people making new claims will have their benefits cut if the government assesses that their condition might improve, charities have said.

In April, the health element of universal credit – an extra payment for people assessed as too unwell to work or prepare for work – will be halved to £50 a week and frozen for new claimants unless their condition is found to be terminal or severe and lifelong with no prospect of improvement.

Ministers had pledged last summer that the “severe and lifelong” clause – known as the severe conditions criteria – would shield the most severely disabled and ill people from the new lower benefit rate.

But charities and disabled people’s organisations (DPOs) have told the Guardian that a wide range of debilitating conditions may not meet the strict eligibility criteria, despite them often leaving someone unable to work. This includes multiple sclerosis, learning disabilities, bipolar, Parkinson’s, ME and long Covid.

Samuel Thomas, a senior policy adviser at the anti-poverty charity Z2K, said: “Cuts to universal credit’s health element threaten to push some of the most seriously ill and disabled people in the country to the brink. The higher rate of support will only be available to [new claimants] whose condition is lifelong and shows no prospect of improvement.

“That rigid test could exclude people who have had strokes or heart attacks, or people with conditions like cancer or schizophrenia. Seriously ill and disabled people facing years of significant difficulty are at risk of being left on a lower rate simply because recovery, however partial or distant, cannot be ruled out.”

Ella Smith, the welfare rights policy lead for the ME Association, said: “The government says it’s protecting people with the most severe conditions, but in reality, this actively excludes swathes of severely ill and disabled people.

“The fact someone might improve at some point in the future should not be used to reduce essential support today. Reassessment already exists if someone’s condition genuinely improves. Awarding people a lower rate just because they may hypothetically recover later turns the constant struggle with unpredictable conditions into an additional cruel penalty.”

Government data estimates that by 2029-30, 730,000 future recipients of universal credit will miss out on the higher rate, with an average loss of £3,000 per year.

While a Labour backbench rebellion last summer paused the proposed personal independence payment reforms, MPs voted to go ahead with changes to universal credit. Under the changes, existing claimants will continue to receive £97 a week whether their disability is judged to be lifelong or not.

James Taylor, the director of strategy at Scope, said: “Essentially, we’ll have a two-tier universal credit system. Two people with a similar condition and similar extra costs will get different amounts. Creating the severe conditions criteria is designed to protect some new claimants but it could add another level of jeopardy for disabled people. There’s a risk that certain conditions might not be judged as ‘severe’ and assessors inconsistently interpret and apply new criteria for a new category.”

There is further concern among charities that to qualify for the higher “severe” payment, new claimants must provide extensive medical evidence from the NHS. This can be harder for certain conditions, particularly at a time when waiting lists are still long.

Hannah Nicholls-Harrison, a policy manager at Mencap, said the rule could mean a significant number of new universal credit claimants with a learning disability would lose out on support.

“Many people struggle to achieve a formal diagnosis of learning disability due to the lack of a clear pathway in the NHS,” she said. “For people with a learning disability who do not have that level of documentation, or who do not have the support to navigate a complex system and advocate for themselves, this will mean missing out on [thousands of pounds] a year.”

Several charities and DPOs said they were concerned the lower benefit rate would push already struggling disabled people into deeper poverty, and in some cases, “destitution”.

Thomas said: “The human cost is stark. Families losing out on this vital income could face eviction, go without food and heating, and lose access to the care they depend on.”

Tom Pollard, the head of policy at Mind, said: “[Government] protecting a higher rate of payment for people meeting the severe conditions criteria is recognition that the reduced rate is not sufficient for people who are out of work for longer periods of time.

“Many people with severe and long-term mental health diagnoses, such as bipolar disorder or schizophrenia, may not meet these criteria, despite impairments and barriers that will often keep them out of work for long stretches of their lives.”

A DWP spokesperson said the government was committed to supporting vulnerable people. They added: “Under the severe conditions criteria, claimants are assessed not on their condition itself but on how it affects them, with the criteria intended to protect those who are not expected ever to be able to work due to a severe lifelong health condition or disability that is unlikely to improve.

“Those with the most severe, lifelong conditions, those nearing end of life, and all existing universal credit health claimants will continue to receive the higher rate.”