On the day that one of the prime minister’s most senior aides cheerily emailed more than 100 staff to suggest drinks in the Downing Street garden, a cabinet minister was telling the public they could meet only one person outside their household in an outdoor public place, 2 metres apart.

The starkly different advice from Oliver Dowden at the No 10 press conference was less than an hour before the email from Martin Reynolds was sent on 20 May 2020. And that disparity would seem to encapsulate better than perhaps any other alleged No 10 party – and there were many – the “one rule for them” approach that has been so damaging for Boris Johnson.

It puts paid to the different explanations No 10 has offered for each new scandal, and it leaves the prime minister most personally exposed.

After the news of the 2020 Christmas party story broke six weeks ago, Johnson himself said he had not attended and that he had been assured by aides the party did not take place.

Many No 10 insiders believed that was to give himself scope to sack the senior aides who briefed him before his PMQs appearance before Christmas, should the heat become too great for him.

When he was revealed to have attended a winter leaving do and a festive Zoom quiz, he argued the approaches were within the rules.

That was the response also taken to the Guardian’s picture of Johnson in the Downing Street garden on 15 May 2020, relaxing with cheese and wine in the company of his wife, Carrie, and their newborn son, as well as Dominic Cummings and Reynolds, his principal private secretary. Despite the aides standing around with glasses, No 10 insisted it was work-related, backed up by Cummings this week – not one to normally cut the PM any slack.

And however unbelievable that explanation seemed when compared with the atmosphere in the Guardian picture, the newly revealed Reynolds invitation makes it ever clearer: that explanation will not wash this time.

Two defences no longer apply. Firstly, Johnson is said by multiple sources to have been in attendance at the 20 May drinks, meaning there is no distance between him and the lockdown-breaking, and no aides can be blamed for misinformation.

Secondly, this gathering cannot be justified as an important work meeting. Reynolds’ jaunty tone makes that clear: “After what has been an incredibly busy period it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No 10 garden this evening.”

Socially distanced they may have been, but the sheer number involved is a clear breach of pandemic rules in place at the time. The tone will be particularly galling as Matt Hancock, the then health secretary, had warned in one of his addresses: “This weekend, with the good weather and the new rules … don’t take risks.”

No 10 has a holding position it can take to avoid commenting before the inquiry by senior civil servant Sue Gray concludes. Reynolds has been widely rumoured to be on the way out, heading back to the diplomatic service having previously been ambassador to Libya for six months before Johnson asked him to return as his principal private secretary. One Whitehall source said he was hoping to return to the Middle East.

Tory MPs had hoped after new year that the heat would die down on the parties scandal, with the public having been allowed to enjoy a more normal Christmas with loved ones. But the new revelations could prove even more damaging, leaving the prime minister with no shield for his own rule-breaking.