The SNP's Westminster leader has slammed Boris Johnson's claim no one told him a drinks gathering at Downing Street was against the rules as a "pathetic excuse".
Ian Blackford told MPs at Prime Minister's Questions that attempts to defend Boris Johnson, referred to as "operation big dog", had become "operation dog's dinner".
He called on the Prime Minister to resign.
Boris Johnson responded by saying it was important to wait for the results of the inquiry into Downing Street drinks gatherings, led by civil servant Sue Gray.
The Prime Minister said Mr Blackford was "wrong in what he asserts".
Boris Johnson also declined to say whether he agrees Scottish Conservative leader Douglas Ross is a "lightweight".
Jacob Rees-Mogg made the comment last week, after Mr Ross called for the Prime Minister to resign.
Boris Johnson did say he believes the Scottish Conservatives do an "excellent job".