U.K. government borrowing surged in September to the highest amount in five years, piling more pressure on Treasury chief Rachel Reeves over the nation's public finances ahead of her fall budget.

Public sector net borrowing rose to 20.2 billion pounds ($27 billion) in September, 1.6 billion pounds higher than the same month last year, the Office for National Statistics (ONS) said on Tuesday.

Most economists were expecting borrowing to total 20.8 billion pounds in September, fractionally more than the 20.1 billion pounds the U.K.'s independent fiscal watchdog, the Office for Budget Responsibility (OBR), forecast in March.

Borrowing in the current financial year, between March and September, was nearly 100 billion pounds – the second-highest amount since monthly records began in 1993, the ONS said.

"Last month saw the highest September borrowing for five years," ONS chief economist Grant Fitzner said.

"Debt interest, the cost of providing public services and benefits all increased compared with last year, more than offsetting the rise in receipts from central government taxes and national insurance contributions," he added.

"Likewise, the first six months of the financial year saw the highest overall deficit since 2020."