Former Downing Street adviser Dominic Cummings said a “senior No 10 official” invited people to “socially distanced drinks” in the garden in May 2020.
In his blog, Cummings wrote that he and another adviser had warned that this could be against the rules but discovered the event had gone ahead.
The BBC understands that the event will be investigated by a senior civil servant.
The first lockdown was introduced in England on March 23 and were slowly relaxed in May 2020, but it was not until 1 June that people were allowed to meet outside in groups of up to six people.
At this time, Cummings was also enraptured in scandal. He caused public outrage when it was reported that he travelled to his parents’ farm in Durham during lockdown while experiencing symptoms, also in May 2020.
45 Tory MPs called for his resignation and he was criticised by opposition parties and the public for his noncompliance with restrictions.
The Prime Minister eventually came out to defend his advisor, stating Cummings had acted “responsibly, legally and with integrity”.
On top of this, Cummings was photographed at another event also in May 2020 and at Downing Street.
The Guardian’s leaked image showed the prime minister with his wife and 17 other staff members – including Cummings – with bottles of wine and a cheeseboard.
Nevertheless, the former adviser emphasised that this event did not break any rules.
“Many meetings had been taking place in the garden all day… those on the terrace nearest me in the photo had been having a meeting.
“No 10 staff were encouraged to have meetings in the garden April-August for the obvious reason that we were in a pandemic with an airborne disease and being outside was safer!” he said.