Boris Johnson has insisted his mid-pandemic birthday gathering that earned him a fine was “reasonably necessary for work purposes”.

The former prime minister also defended the presence of luxury interior designer Lulu Lytle because she was a “contractor” working in No 10.

Mr Johnson, his wife Carrie and then-chancellor Rishi Sunak all received £50 fixed-penalty notices from Scotland Yard for the 56th birthday bash in the Cabinet Room.

Questioned by MPs on the Privileges Committee, Mr Johnson said he was joined by his “wife and son and, yes, there was a contractor who was working in the building”.

I thought it was a completely innocent event

Ms Lytle had been working on the lavish renovations of the official flat that Mr Johnson and his family were living in.

“I had come back from a long external work visit, I thought it was reasonably necessary for work purposes because I was standing at my desk surrounded by officials who had been asked to come and wish me happy birthday,” Mr Johnson told MPs.

“I had only just recently recovered from an illness, from Covid, and it seemed to me to be a perfectly proper thing to do.

“We were about to have another meeting and they were very largely the same officials.”

He said Mr Sunak, now Prime Minister, would have been “just as surprised as I was” about the fines they received.

“I thought it was a completely innocent event,” Mr Johnson said. “It did not strike me as anything other than an ordinary common or garden workplace event.”

Mr Johnson insisted the birthday gathering consisted of only mild festivities.

He added: “No one sang, the famous Union Jack cake remained in its Tupperware box, unnoticed by me, and was later discovered and eaten by my private secretaries.”

He said a “slightly exaggerated” version of the event was briefed to The Times “with singing and cake eating” and yet nothing untoward was detected “either by the reporter or by millions of eagle-eyed readers”.