Millions are at risk of a benefits "lock out" because of the current Universal Credit set-up, The Salvation Army has warned.
It fears that up to two million sickness benefit claimants could struggle to access the digital system, leaving them unable to pay their rent or buy food.
It wants the system simplified to make it easier to make a claim.
The call comes after research by The Salvation Army found that 85% of Universal Credit claimants struggled to complete their claim.
Of these, nearly two thirds (60%) said this was because they were not able to use a computer or did not understand the system.
Rebecca Keating, Director of Employment Services at The Salvation Army said: "Rolling out Universal Credit in its current form will steamroll vulnerable people into poverty but the Government has time to turn this around by accepting our recommendations and making it easier to apply.
"Millions of people need extra support accessing a computer or understanding how to fill in complicated online forms. It is these vulnerable people who also claim Employment Support Allowance (ESA), a benefit for those who need extra help to get back into work.
"Over two million people are currently claiming ESA and are due to be moved onto Universal Credit<sup>3. Our research shows that many of them are going to struggle to access a system that is complicated, bureaucratic and digital by default."
The Salvation Army is calling for better support, particularly for those with mental health issues, and a reduction in the caseloads handled by Jobcentre Work Coaches so that they have more time to identify and support clients who need extra help.
"Universal Credit is already the main reason people are coming to our food banks," Keating continued.
"They come to us for help at the point when they have given up and got themselves into debt trying to manage without the money for rent and food.
"We are helping those we can but the system is complicated even for those who are not classed as vulnerable and are applying for the standard Job Seekers Allowance who may also struggle.
"Half of people we surveyed said that mental ill health meant they struggled to move on to Universal Credit. The Government needs to seriously rethink the implications of what that means for moving more people onto the benefit."
