Britain's Electoral Commission on Wednesday announced a formal investigation into how Prime Minister Boris Johnson paid for a lavish makeover of his Downing Street flat, seriously escalating a simmering scandal.

"We are now satisfied that there are reasonable grounds to suspect that an offence or offences may have occurred. We will therefore continue this work as a formal investigation to establish whether this is the case," the watchdog said.

Johnson later denied wrongdoing, saying he paid the refurbishment personally.

"The answer is I have covered the costs and... of course there's an electoral commission investigating this and I can tell him I have conformed in full with the code of conduct and ministerial code, and officials who have been kept been advising me throughout this whole thing," he told parliament shortly after the formal investigation against him was announced. 

(With input from AFP and Reuters)

(Cover: Britain's Prime Minister Boris Johnson holds a news conference at 10 Downing Street, amid the COVID-19 outbreak, in London, Britain, April 20, 2021. /Reuters)