BORIS Johnson appears to have broken the law by hosting a Christmas Quiz in Downing Street last year during lockdown, Sir Keir Starmer has said.

The Labour party leader, a former director of public prosecutions, said Mr Johnson was “unfit for office” and a "threat to public health" who had lost the public's trust.

“He’s the worst possible leader at the worst possible time,” he told BBC One’s Andrew Marr Show.

However Sir Keir stopped short of calling for Mr Johnson’s resignation - a reflection that Labour is benefiting from the Tory chaos and scandals of recent weeks.

A new poll today puts Labour nine points ahead of the Conservatives, the biggest gap since 2014.

The Opinium survey for the Observer also found 57 per cent of voters thought Mr Johnson should resign, up nine points in just a fortnight.

The Sunday Mirror today printed a picture of Mr Johnson hosting a virtual Christmas Quiz on December 15 last year, when indoor social gathering were banned in London.

The PM was flanked by two staff, one in a party and one in a tinsel scarf, in the picture, with reports of larger groups of staff taking part in other rooms at Number 10.

At the time, London was under Tier 2 restrictions, meaning indoor gatherings of two or more people were banned.

Downing Street said Mr Johnson “briefly took part in a quiz” to thank staff for their hard work during the pandemic.

Two other Christmas parties in Downing Street are currently being investigated.

Asked if he thought Mr Johnson was breaking the law, Sir Keir said: “Well, it looks as though he was.

“He must have known that other groups were in other rooms in his own building.

“This is very important because he has damaged his authority, he is now so weak, his party is so divided – he can’t deliver the leadership this country needs.

“We’ve got a very important vote coming up next week and he can’t even discharge the basic functions of Government. He is the worst possible leader at the worst possible time.”

The quiz allegations were “very serious”, he said.

“The reason it is serious is because at the time the Government was asking the public not to see loved ones last Christmas at all.

“Many of them didn’t see their loved ones again, and that is why it has created such fury.

“This is the latest of the allegations and remember it was only on Wednesday that the Prime Minister stood up in Parliament and said he was furious that he had just found out that there may have been breaches of the rules in Downing Street.

“ He was talking about the party on December 18.

“Now we learn that three days earlier he was involved, he was leading a quiz in Downing Street which, if the reports are accurate, involve groups of people – I think it was being said up to 24 people in one room – using their computer screens but actually involving themselves in a social event.

“So he will be doubly furious now he finds that he himself has been involved in something that may have breached the rules.”

He gave a pledge that Labour had been following the rules.

He also said Labour would back the introduction of tougher coronavirus measures south of the border, sparing Mr Johnson from a potentially lethal rebellion.

Mr Johnson is facing the biggest revolt of his premiership on Tuesday, as MPs are asked to support the Plan B restrictions as the Omicron variant sweeps the country.

Around 60 Tory MPs are expected to oppose Covid passports for nightclubs and other venues, more mandatory face coverings and tighter self-isolation rules.

It means Mr Johnson’s plan would be defeated without opposition support.

Despite the opportunity to destabilise the Prime Minister further, the Labour leader said his MPs would support the Plan B changes as necessary for the public good.

He said: “Throughout the pandemic we have acted in the public interest.

“We will be voting for the measures on Tuesday to protect the NHS and protect the public.

“We are not going to be tempted to play politics here.

We will act in the public interest.”

On Sky News, Education Secretary Nadhim Zahawi insisted the quiz was “virtual” and dismissed criticism about parties as “hype”.

He said the Sunday Mirror picture showed a “Prime Minister on a virtual quiz night for 10-15 minutes to thank his staff”, who he said had no choice but to come into work.

He insisted Mr Johnson had been “respecting the lockdown rules” and that the other people pictured in the quiz were in his office people who “work with him”.

LAter, on Marr, Mr Zahawi again said the reporting of the parties had been "hype", and that the Cabinet Secretary would investigate all the allegations.

He also accused Sir Keir Starmer of playing politics on the issue, despite claiming he wasn't.

Pressed on how Tier 2 restrictions were “not broken” in the image of Mr Johnson taking part in the quiz flanked by two colleagues, Mr Zahawi said: “These are people who work together.

“There is no drink here. I don’t think there is a rule against recognising Christmas with tinsel or a hat.

“Ten to 15 minutes to thank his team who were working all hours on a virtual screen in a pub quiz – I think people can make their own mind up on that.”

Asked if he now accepted there was a party in No 10 on December 18 following leaked footage of aides joking about a festive gathering only days later, the Education Secretary replied: “I think that is unfair.

“Allegra Stratton, who resigned over this, she resigned because she felt the frivolity and the laughter… the Cabinet Secretary is looking at all allegations.

“I think it is only fair and right, because these are the values that we adhere to in this country, that we should wait for that investigation.”