A Conservative MP's claim that an alleged Downing Street lockdown party did not break the rules as attendees "had worked incredibly hard" and "did not increase the risk of contagion" has been described as "embarrassing".
And a legal expert said the argument made by Michael Fabricant, the Tory MP for Lichfield, "doesn't really stack up'.
ITV News published an email invite on Monday asking No 10 staff to attend a “bring your own booze” party in the Downing Street garden on 20 May 2020, during the first COVID-19 lockdown.
It is reported that about 40 staff attended the party, including the prime minister and his wife Carrie Johnson.
On Tuesday, Fabricant defended Downing Street, tweeting that staff "would not have increased the risk of contagion" and that they had "worked incredibly hard".
Later, he told BBC News: "The garden is an extension of their workplace and there was no mixing with outside people."
Human rights lawyer Adam Wagner, who appeared on the same channel immediately after Fabricant, said: "At that time it was illegal for a person to be outside a place they were living without a reasonable excuse.
"It doesn’t really stack up, any of the arguments Mr Fabricant was making. The guidance at the time said to avoid gatherings in the workplace.
"It was probably bringing together lots of people who weren’t working regularly together, who were working in different areas, in a way which would potentially expose them to COVID.
"This was a recipe for a disaster really to bring together all these people."
Labour MP Dianne Abbott tweeted that Fabricant had made an “embarrassing defence” of the Downing Street party.
On Monday, Philip Dunne, the Tory MP for Ludlow, also defended Downing Street staff, saying they were having a "drink after work".
He told BBC's Newsnight on Monday: “The other thing to bear in mind is this was three months after the most intensive period of crisis management that any government has faced in peacetime."
Watch: Tory MP defends Downing Street staff having 'drink after work'
The email invite said staff should “make the most of the lovely weather”, ITV reported, despite England being under tough coronavirus restrictions banning groups from meeting socially outdoors when the message was sent.
A spokesman for Scotland Yard said: “The Metropolitan Police Service is aware of widespread reporting relating to alleged breaches of the Health Protection Regulations at Downing Street on 20 May 2020 and is in contact with the Cabinet Office.”
Johnson refused to answer questions on Monday evening about the alleged party and failed to attend a debate on the issue in parliament.
Number 10 said it would not be commenting on the allegations while civil servant Sue Gray carries out her inquiry into numerous allegations of rule-breaking events being held in Downing Street during the coronavirus pandemic.
Gray will also examine an alleged gathering in the Downing Street garden on 15 May 2020, revealed by a leaked photo showing Johnson and staff sitting around a table with cheese and wine.
Hannah Brady, a spokeswoman for the COVID-19 Bereaved Families for Justice Group, said her father Shaun, who worked at the Kraft Heinz factory in Wigan, died four days before the email by Reynolds was sent out.
Brady said her father was 55 and fit and healthy when he contracted coronavirus, and she and her family had done “everything they could” to keep him safe during lockdown.
“Those days will stay with me for the rest of my life, just like the families of the 353 people that died that day, my family couldn’t even get a hug from our friends,” she said.
“To think that whilst it was happening Boris Johnson was making the ‘most of the weather’ and throwing a party for 100 people is truly beyond belief."
Watch: Boris Johnson declines to answer if he attended Downing Street party