Sajid Javid has resigned as the head of the U.K.'s finance ministry, surprising financial markets and political pundits in the country.

He has served as finance minister (known as the Chancellor of the Exchequer in the U.K.) since July, when Boris Johnson was first appointed prime minister. He will be the shortest serving U.K. finance chief since 1970.

Javid — the son of a Pakistani bus driver who previously served as home secretary — was due to present Britain's budget in March, the U.K.'s first after its departure from the EU last month. The resignation comes as Johnson was making changes to his Cabinet on Thursday where the U.K.'s attorney general, Geoffrey Cox, also resigned.

Media reports suggest that Javid was offered the chance to remain finance minister on the condition that he fires all of his advisors. He reportedly said no and opted to resign himself.

There had been growing tensions between the finance minister and the prime minister's inner circle, in particular ahead of the budget release in four weeks' time. According to the Financial Times, allies of the prime minister dubbed Javid "Chino," meaning chancellor in name only.

Rishi Sunak, who has served as chief secretary to the Treasury since July 2019, will step into the role.