LONDON, Jan 20 (Reuters) - An investigation into Downing Street parties has found an email from a senior official to Prime Minister Boris Johnson"s private secretary warning that a May 20, 2020 party should not go ahead, ITV"s political editor said on Thursday. "I understand Sue Gray has found the email from a senior official to PM’s principle private secretary Martin Reynolds warning him the 20 May party should not go ahead," Robert Peston said on Twitter. Johnson denied on Tuesday an accusation by his former adviser that he had lied to parliament about a lockdown party, saying nobody had warned him the "bring your own booze" gathering might contravene COVID-19 rules. Johnson had already last week apologised to parliament for attending the May 20, 2020 gathering in the Downing Street garden. He said he was there for 25 minutes to thank staff. But Dominic Cummings, an architect of Britain"s departure from the European Union and a former senior adviser who left government under acrimonious terms in November 2020, said that Johnson had agreed the drinks party should go ahead. Cummings said that he and at least one other adviser told Reynolds that it should be cancelled. Asked if he had lied to the public and parliament, Johnson told reporters on Tuesday: "No. Nobody told me that what we were doing was, as you say, against the rules ... I thought that I was attending a work event."