Lockdown rules when Boris Johnson attended the drinks do in the No 10 garden barred people from leaving their homes without “reasonable excuse” while official guidance said they should “minimise” workplace gatherings.

The Prime Minister has said he believed the gathering on May 20 2020 had been a “work event” and may have “technically” been permitted within the rules.

However some lawyers have questioned his explanation, saying it was hard to argue that a social event was “necessary” for work purposes.

A week before No 10 staff were invited to “bring your own bottle” to the Downing Street garden, the rules for England had been significantly eased for the first time since the country went into lockdown in March 2020.

In particular, restrictions on outdoor exercise were lifted and some strictly limited outdoor mixing with one other person from another household was allowed provided two metres social-distancing was maintained.

However many of the original regulations remained in place with the regulations setting out in law what counted as a “reasonable excuse” for people to leave their homes.

They included going to work “where it is not reasonably possible for that person to work … from the place where they are living”.

At the same time the rules also prohibited by law all gatherings in public of more than two people except for reasons set out in the regulations – including if it was “essential” for work purposes.

Government guidance issued alongside the regulations stated that “workers should try to minimise all meetings and other gatherings in the workplace”.

Adam Wagner, a human rights barrister who has spent the pandemic interpreting complex coronavirus laws and explaining them to the public on social media, said it was hard to see how the Downing Street event could be justified under the rules in place at the time.

“The law in place at the time focused on the need to work as a reasonable excuse to leave the house, so the work would have to be necessary,” he told the PA news agency.

“Anything could be classed as a work event but it is difficult to see how a social gathering would be necessary for work.

“No work is being done and it could be reasonably be done by other means from home.”