Around 30,000 people with mental illness will spend Christmas with the prospect of a tribunal hanging over them, having attempted to claim disability benefits, campaigners have said.
Analysis of Government figures by Rethink Mental Illness suggests some will spend the festive period missing out on upwards of £100 a week, with the majority of appeals being overturned.
The charity estimates more than 15,000 people with mental illness will be waiting for a tribunal hearing to claim Employment and Support Allowance (ESA).
Almost 14,000 will be waiting for a similar hearing over Personal Independence Payment (PIP), according to its analysis.
Around two thirds of appeals for ESA and PIP were overturned in the claimant's favour in 2016/17, according to Ministry of Justice figures.
The Department for Work and Pensions says a small fraction of claims are appealed and most are overturned because claimants submit new evidence.
Brian Dow, director of external affairs at Rethink Mental Illness, said: "Christmas is a time to celebrate, but tens of thousands of people with mental illness will be spending it without the money that they should be entitled to, trapped in a long and stressful appeals process.
"The money from these benefits can be a lifeline. It keeps food in the fridge, bills paid, and allows people to leave the house and be part of their community.
"The fact that two thirds of these decisions will be successfully overturned reflects a system that cannot accurately assess and manage mental illness.
"Thousands of people go through a lengthy, stressful process that can wreak havoc with their mental health, only to come out with an incorrect decision that takes four months to appeal."
Rethink Mental Illness says many awaiting a tribunal on Christmas Day will have been waiting since the summer, with average waiting times for tribunal decisions reaching 18 weeks earlier this year.
The analysis coincides with a major new report from the charity, which found that 38% of claimants felt that delays in decisions led to a deterioration in their mental health.
Some 19% had to take higher doses of medication in order to cope with the increased stress.
One person told the charity they were left waiting nine months to get to the tribunal stage.
"We need a fundamental overhaul of how mental heath is assessed," Mr Dow said.
"The Government has let too many opportunities pass by and if it is really serious about creating a better society for people with mental health this has to be a priority."
The charity is calling for an overhaul of the assessment process for both PIP and ESA, and for those with the most severe mental health conditions to be exempt from face-to-face assessments.
It has also called for all assessors and DWP decision makers to be appropriately trained in mental health, with previous concern such conditions were being assessed by physiotherapists.
Some 524,000 people claiming PIP have a psychiatric disorder as their main condition - around a third of all claimants.
Around half those claiming ESA do so due to mental and behavioural disorder.
Last month Sir Ernest Ryder, senior president of tribunals, said the quality of evidence provided by the DWP in benefits cases was so poor it would be "wholly inadmissible" in any other court.
A DWP spokeswoman said: "Assessments are an important part of PIP and ESA to ensure that people get the right level of support.
"Only a small proportion of all decisions are overturned at appeal - just 4% of PIP and 4% of ESA.
"However, we continuously review our processes to ensure they're working in the best way possible, including ending ESA reassessments for people with the most severe conditions."
Ministers say that PIP looks specifically at how someone's life is affected by mental health, unlike the old system which did not sufficiently recognise mental health problems.
There are now more people with a mental health condition receiving the higher rates of both PIP components.
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