A leaked e-mail reveals 100 people were invited to 'bring their own booze' to the Number 10 garden in May 2020.
Boris Johnson and his wife Carrie are understood to be among around 40 who attended.
Health minister Edward Argar says the government won't comment while Sue Gray's independent investigation's ongoing.
Former Croydon Central MP, Lord Gavin Barwell says “it is not entirely clear why the PM needs to wait for Sue Gray’s report to find out if he went to a party in his own garden”.