Government borrowing was higher than expected at £18bn last month, according to official figures.
The Office for National Statistics said it was the highest August borrowing figure for five years.
Most economists had expected government borrowing to come in at £12.8bn.
Borrowing for the first five months of the financial year was £83.8bn – £16.2bn higher than the same period a year ago.
The figure will cause a headache for Chancellor Rachel Reeves, who is already expected to announce tax increases at the autumn budget in November to
stay on track to meet her fiscal rules and avoid unsettling
financial markets.
ONS chief economist Grant Fitzner said: “Although overall tax and National Insurance receipts were noticeably up on last year, these increases were outstripped by higher spending on public services, benefits and debt interest. Total borrowing for the financial year to date was also the highest since 2020.”
James Murray, the chief secretary to the Treasury, said the government has a “plan to bring down borrowing” to ensure taxpayer money is spent on the country’s priorities.
“Our focus is on economic stability, fiscal responsibility, ripping up needless red tape, tearing out waste from our public services, driving forward reforms and putting more money in working people’s pockets,” he said.
Analysts have warned the figures mean tax rises at the autumn budget are now inevitable.
Martin Beck, chief economist at WPI Strategy, said: “The £10 billion buffer the Chancellor pencilled in against her key fiscal rule in March has almost certainly gone. That means tax rises in November look inevitable.”