Government borrowing reached £11.8bn last month, twice the level forecast by the fiscal watchdog.
Soaring inflation led interest payments on UK government debt to hit a new record in August.
Interest payments reached £8.2bn during the month, £1.5bn more than last year and the highest since monthly records began in 1997, the Office for National Statistics (ONS) said.
The high interest payments come as the government borrows billions of pounds to help households with energy bills.
It will also reportedly unveil £30bn of tax cuts in a mini-Budget on Friday.
The ONS said the government had borrowed £11.8bn in August, £2.6bn less than August 2021, but £6.5bn more than before the pandemic two years ago.
Martyn Beck, chief economic advisor to the EY Club and a former economist at the Treasury, said although borrowing had fallen, it was higher than expected.
The recent high levels of debt interest payments are largely a result of higher inflation, the ONS said.
This is due to the interest paid on government bonds rising in line with the Retail Prices Index measure of inflation, which hit 12.3% in August.