Britain's government ran a budget deficit of £23.3 billion pounds ($30.7 billion) in May, the Office for National Statistics said on Friday, up 30% on a year earlier and above all economists' expectations in a Reuters poll.
Economists polled by Reuters had a median forecast of £18.5 billion for the month.
In March, before the impact of the US-Iran war was clear, the government's budget watchdog forecast Britain would run a £115.5 billion deficit in the 2026/27 financial year, equivalent to 3.6% of national income and down from 4.3% in 2025/26.
Since then, the outlook for growth has weakened and borrowing costs have risen.
Last week Britain offered investors the highest yield since at least 1998 when it sold £9 billion of 15-year debt.
The government is also struggling to finance extra defense spending within existing budget rules and previous promises to other departments, prompting defense minister John Healey to resign in protest last week.
