The UK borrowed £23.3bn in May, according to official figures, up almost a third on the same month last year.
May's borrowing figure — the difference between spending and income from taxes — was £5.6bn higher than forecast by the Office for Budget Responsibility (OBR), the independent fiscal watchdog.
The OBR forecast was made in March, at which point the impact of the war in the Middle East had not yet become clear.
The Office for National Statistics (ONS) said interest payable on government debt jumped to £11.7bn – the highest ever recorded in any May.
The cost of borrowing for governments has shot up during the conflict, while the surge in energy prices since the beginning of the Iran war has pushed up inflation.
A peace agreement between the US and Iran has led oil prices to drop significantly, but economists have warned that the knock-on effects of the conflict are still on the way.
The figures came as Andy Burnham was elected MP for Makerfield in a by-election, paving the way for him to launch a leadership challenge against Keir Starmer.
Capital Economics said the higher-than-expected borrowing highlights "the fragile fiscal backdrop that will face whoever is in 10 Downing Street."
"This will constrain whoever is Prime Minister following a potential leadership challenge after Andy Burnham's win in the Makerfield by-election."
