Universal Credit claimants are being "blocked" from challenging decisions they disagree with, a campaign group has claimed.
The system is throwing up so many obstacles for those seeking reviews of decisions that some give up, said the Child Poverty Action Group (CPAG).
An analysis of 1,600 benefit cases suggested that some claimants who suspect the Department for Work and Pensions (DWP) has made a mistake are wrongly advised that decisions cannot be appealed.
The DWP said it has already improved guidance and advice to staff.
The CPAG claimed efforts to have mistakes corrected are being delayed or blocked because of wrong advice given to claimants.
When people have a first claim refused, their online account is often closed, making it more difficult to have decisions reviewed, said the group.
CPAG chief executive Alison Garnham said: "The failure to ensure Universal Credit operates in a way that upholds basic legal duties is cause for serious concern.
"UC staff dealing with claimants do not always seem to understand the rules as to how decisions can be challenged, and efforts to make the system more user-friendly by encouraging informal online chats can mean claimants are prevented from exercising their rights and ultimately cannot make sure their awards are corrected.
"The system throws up so many obstacles to getting a decision reviewed that some claimants - often the most vulnerable - are likely to give up and lose out.
"One hundred and thirty thousand individuals and families are moving on to Universal Credit each month. If it isn't accountable, and if appeal routes are not crystal clear and readily available to claimants, then Universal Credit isn't fit for purpose."
A DWP spokesman said: "We continue to work closely with CPAG and welcome the opportunity to do so. We have already improved guidance online and advice to staff about mandatory reconsiderations.
"Anyone who disagrees with a benefits decision is able to request a mandatory reconsideration either online, by phone, in person or in writing."
