THE UK Government underpaid people on benefits by £2.6 billion in 2021/22.
New figures also show the Department for Work and Pensions (DWP) overpaid £8.6bn - 4% of the total benefit expenditure - as a result of their own mistakes and fraud.
The staggering data comes after Chancellor Rishi Sunak set out a package of measures on Thursday to address an ever-worsening cost-of-living crisis.
SNP MP Joanna Cherry branded the £21bn package a "diversion tactic" from the Tories to distract from the damning findings of the Sue Gray report into lockdown parties.
And the party has now blasted Sunak's pledge to help those who need it most as "hollow" and insisted the DWP "needs to get its act together".
Kirsty Blackman, Work and Pensions spokesperson, said: "The Tory government’s talk of targeting financial support for those that need it most rings hollow when we see that those very same people have missed out on almost £3bn immediately after a pandemic and amidst a cost of living crisis.
“The Department for Work and Pensions needs to get its act together - we cannot afford errors or mistakes that lead to millions missing out on vital cash at a time when people are struggling to afford the very basics.
"It could have made all the difference to someone who was having to choose between eating and heating their home. "The UK Government must take urgent action to ensure those who have been underpaid get the money they are entitled to as soon as possible, and those who have been overpaid should not be forced to pay it back. “The Chancellor stood up in Parliament yesterday and spoke of targeting support to the poorest. However, it is temporary and only a sticking plaster if they will not sort out the current welfare system so that people get the money they are owed and on time, and if they will not bring forward a long-term strategy to help people through the cost of living crisis."
The party has called for the five-week wait for claimants' first Universal Credit payments to be scrapped and for the £20-per-week cut to be reversed, while SNP MPs believe the bedroom tax and two-child cap also need to go.
Blackman added: "Uprating benefits and bringing in a Child Payment - as the Scottish Government has done - would make a real difference to families everywhere.”
Sunak announced every household in the UK would get £400 in October to help them with an expected further hike in energy bills.
The poorest households are also set to receive a one-off payment of £650 paid directly into their bank accounts in two lump sums in July and the autumn.
The new measures will also include separate one-off payments of £300 for pensioner households and £150 for individuals receiving disability benefits.
The cost will be partly offset by a 25% windfall tax on oil and gas firms' profits.
