British Prime Minister Rishi Sunak has fired the chairman of his ruling Conservative Party over a “serious breach” of the ministerial code.

Pressure had been building on Nadhim Zahawi amid allegations he settled a multimillion-dollar unpaid tax bill while he was in charge of the country’s Treasury.

In a letter to Mr Zahawi on Sunday (local time), Mr Sunak wrote he had been forced to act after promising at the start of his tenure that his government “would have integrity, professionalism and accountability at every level”.

There was no immediate comment from Mr Zahawi.

Mr Zahawi, the founder of polling website YouGov, had acknowledged a dispute with tax authorities, but argued his error was “careless and not deliberate”.

The British media reported the settlement came to almost £5 million ($9 million).

Mr Zahawi headed the UK Treasury from July to September 2022 in the final months of Boris Johnson’s time as prime minister.

Mr Sunak said the decision had come at the end of an investigation into the party chairman’s financial dealings by the government’s independent standards adviser.

The report by Laurie Magnus found Mr Zahawi had shown “insufficient regard” for the ministerial code and the required standards “to be honest, open and an exemplary leader through his own behaviour” in public life.

The investigation into his affairs by HMRC, Britain’s tax office, centred on the sale of some £27 million in shares in YouGov.

The probe began in April 2021, but Mr Zahawi did not declare it when he was appointed chancellor of the exchequer more than a year later.

The Magnus report found there should have been an understanding from the outset the matter was serious.

Mr Zahawi previously worked as vaccines minister in the first year of the coronavirus pandemic, followed by a nine-month stint as education minister.