More than £100 million has been spent by the Department for Work and Pensions on administering reviews and appeals against disability benefits in little more than two years, new figures show.
This is in addition to the tens of millions of pounds spent every year by the Ministry of Justice on the appeals, around two-thirds of which have been won by claimants in the past year.
The bill has been branded "staggering" and prompted a Tory former minister to claim "something is seriously wrong with the system".
DWP said a small proportion of decisions were overturned and most Employment and Support Allowance (ESA) and Personal Independence Payment (PIP) claimants were happy with their assessments.
But the department is also facing questions from the Work and Pensions select committee over the figures, amid claims it was not given similar information for its own inquiry into PIP and ESA.
Figures obtained by the Press Association through a Freedom of Information request show DWP has spent £108.1 million on direct staffing costs for ESA and PIP appeals since October 2015.
The figure covers mandatory reconsiderations, an internal DWP review, and appeals to tribunals run by HM Courts and Tribunals Service.
The monthly cost has been steadily rising and in December DWP spent £5.3 million on mandatory reconsiderations and appeals for PIP and ESA.
The equivalent figure for October 2015 was £2.6 million.
"To spend this amount on admin fighting to uphold flawed decisions that shouldn't have been made in the first place is staggering," said Neil Heslop, chief executive of disability charity Leonard Cheshire.
"Thousands of disabled individuals have had to fight to receive support to which they are legally entitled."
Since October 2015, 87,500 PIP claimants had their decision changed at mandatory reconsideration, while 91,587 claimants won their appeals at tribunal.
In the first six months of 2017/18 some 66% of 42,741 PIP appeals went in the claimant's favour.
The figures for ESA since October 2015 show 47,000 people had decisions revised at mandatory reconsideration and 82,219 appeals went in the claimant's favour.
So far in 2017/18, 68% of 35,452 ESA appeals have gone in favour of the claimant.
Tory peer Baroness Altmann, a former minister at DWP, said the money could be spent on benefits for those who need them, rather than on the costs of fighting claims.
"Disability benefits need an overhaul and, of course, we must not let people make bogus claims, but the extent of the appeals we are seeing clearly indicates that something is seriously wrong with the system," she said.
Figures released to the select committee's inquiry show further costs to taxpayers.
The Ministry of Justice says it spent £103.1 million on social security and child support tribunals in 2016/17, up from £92.6 million the year before and £87.4 million in 2014/15.
Around 190,000 cases were cleared with or without a hearing in 2016/17, it told the committee.
Disability benefits make up the bulk of these tribunals, with 136,052 hearings held for either PIP or ESA claims in the same year, according to MoJ figures.
In a letter to the select committee, then justice minister Dominic Raab said the average cost of an appeal had more than doubled to £579 in 2014/15 because PIP cases "now comprise a much larger proportion of the caseload".
PIP cases require more members on the tribunal panel which increases the average cost, he said.
The select committee is due to publish the results of its inquiry into PIP and ESA on Wednesday.
Chairman Frank Field has written to Esther McVey, the Work and Pensions Secretary, in the wake of the costs figures provided to the Press Association to question why MPs were not given such information.
DWP gave the committee the average costs of a mandatory reconsideration and appeal for PIP and ESA.
However, Labour MP Mr Field said the committee was unable to work out the full cost of the appeals process.
This was because it was told information on PIP appeals was not available on whether they were appeals from new claimants or those being reassessed, which have different costs.
The information released to the Press Association was broken down into costs for new claims and those undergoing reassessments.
"That this data was provided in response to an FOI request, but not for our report, is doubly regrettable since the key theme of our report is the need to introduce much greater trust and transparency into the PIP and ESA systems," Mr Field wrote in his letter to Ms McVey.
A DWP spokeswoman said it was working to improve the process, including recruiting around 190 officers who will attend PIP and ESA appeals to provide feedback on decisions.
"Assessments are a necessary part of the benefits system, and everyone has the right to appeal a decision if they're unhappy," she said.
"A vast majority of people are happy with their assessments, and only a very small proportion of all ESA and PIP decisions are overturned at appeal - just 4% for PIP and 5% for ESA.
"We've already commissioned five independent reviews of the work capability assessment - implementing over 100 of their recommendations - and two independent reviews of PIP assessments.
"Meanwhile, we continue to spend over £50bn a year on supporting people with disabilities and health conditions."
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