Boris Johnson is to face MPs later at Prime Minister's Questions amid demands for him to say whether he attended a drinks event in the Downing Street garden during lockdown.

It will be the PM's first public appearance since it emerged 100 staff were invited to the May 2020 gathering.

Mr Johnson has so far declined to say whether he attended it.

However, a number of Conservative MPs have said this position is unsustainable.

On Monday, ITV published a leaked email from Mr Johnson's principal private secretary, Martin Reynolds, inviting staff to "socially distanced drinks in the No 10 garden" on 20 May 2020 - when lockdown rules in England banned large outdoor gatherings.

The invitation encouraged people to "bring your own booze" and "make the most of the lovely weather".

Watch: "Boris Johnson must now resign," says Liberal Democrat leader Sir Ed Davey

Liberal Democrat leader Sir Ed Davey told BBC Breakfast that if the prime minister had a "shred of decency left", he should resign today.

He added Mr Johnson was "incapable of leading our country through this public health crisis", accusing him of being a "threat to the health of the nation" as the public would not listen to anything he had to say.

Labour's deputy leader, Angela Rayner told BBC Breakfast that if Mr Johnson had broken Covid rules and lied to the public and parliament, then his position as prime minister was now "untenable".

"The prime minister cannot break the law, lie to the British public and not be held to account for that," she added.

'He knows whether he was there'

If the prime minister won't resign, it was the duty of Conservative MPs to "do the right thing" and ensure he does "in the interests of the country", Sir Ed argued.

Mr Johnson has said it is a matter for Sue Gray - the senior civil servant investigating a series of reported parties in Downing Street and in Whitehall in 2020 - to determine what happened.

The PM's official spokesman has also refused to confirm whether Mr Johnson attended the May 2020 Downing Street gathering. He added that the prime minister retained full confidence in Mr Reynolds.

But backbencher Nigel Mills told BBC Newsnight an inquiry was not needed to establish whether the prime minister was there.

"He knows whether he was there or not. Just come out and say what happened," he said.

Mr Mills said it would be "utterly untenable" for any senior figure who attended the gathering to be responsible for setting Covid-19 policy.

"If the prime minister knowingly attended a party, I can't see how he can survive," he said.

Conservative MP Huw Merriman insisted "more clarity" was needed from the prime minister on what took place and whether he attended, adding that people were "in the dark" about what went on.

He told BBC Radio 4's Today programme that he didn't believe Mr Johnson should resign, adding that people should be judged on the positives and negatives, citing government achievements including the successful vaccination rollout and the prime minister "holding his nerve" on refusing to introduce further Covid rules in England.

Scottish Conservative leader Douglas Ross said Mr Johnson must quit if he was found to have broken Covid rules.

Sir Charles Walker, vice-chairman of the Conservative backbench 1922 Committee, said there was a lot of anger over what had happened, adding that the prime minister urgently needed to rebuild public trust.

"I think the prime minister needs to spend the next six months restoring trust in No 10 and making some good and strong decisions. I think that is the challenge for him," he told Channel 4 News.

A DUP MP cries while questioning the government over claims it broke lockdown rules

Labour leader Sir Keir Starmer is expected to return to Prime Minister's Questions later, having missed last week's session after contracting Covid.

The prime minister is likely to face questions at the despatch box he has so far declined to answer, having faced criticism at all levels of his party, from constituency officials to the major donor John Caudwell.

On Tuesday, Mr Johnson sent Paymaster General Michael Ellis to respond on his behalf to an emergency Commons question on the issue tabled by Labour.

The Conservative benches were sparsely populated, while no senior ministers appeared publicly to defend Mr Johnson.

Ms Rayner accused the prime minister of avoiding scrutiny, adding that "his absence speaks volumes, as do his smirks on the media".

She said: "The public has already drawn its own conclusions. He can run, but he can't hide."

Elsewhere in that Commons session, MPs recounted stories of those who had suffered during the pandemic while obeying the Covid rules of the time.

DUP MP Jim Shannon was brought to tears when speaking of the death of his mother-in-law during the pandemic.

But one Conservative who did offer the prime minister her support, Suzanne Webb, said: "The time of the House is better spent at this moment in time, as we recover from this pandemic… debating how we can build back better and level up."

In December, the prime minister asked Simon Case, the UK's top civil servant, to investigate a series of allegations about events taking place in government buildings in 2020 despite coronavirus restrictions being in force.

Mr Case stepped down after suggestions one of the parties had happened in his own office and the investigation was passed to Ms Gray.

There have been a series of allegations over parties held during lockdown in 2020, but Ms Gray's inquiry is investigating specific events including a leaving drinks for a No 10 aide on 27 November, a Department of Education gathering on 10 December and a Christmas quiz for No 10 staff on 15 December.

It is also examining the Downing Street Christmas party on 18 December, and on Tuesday, Ellis said drinks events held in the Downing Street garden on 20 and 15 May 2020 would also be added to the probe.