Shortly after 3:30 p.m. on Jan. 7, Dominic Cummings pressed the trigger on the latest tirade against his former boss, Boris Johnson.

Since being forced acrimoniously from his post as the U.K. prime minister’s most powerful adviser 14 months ago, Cummings has repeatedly resurfaced to throw dirt at Johnson and his government. This time it was a zinger.

In May 2020, a senior official in Johnson’s office had invited staff to a pandemic rule-breaking drinks party in the garden of the prime minister’s official residence and workplace at No. 10 Downing St., Cummings wrote in his blog. Sue Gray, the senior civil servant investigating allegations of other illicit gatherings, should be able to dig up email proof, he said.