Emily Thornberry has accused the Prime Minister of getting away with things "because he's posh".
The Labour MP said his use of humour and "extraordinary noises" were no longer funny, as tensions soar in Parliament.
"He [Boris Johnson] thinks he can get away with things by being funny and by using a good turn of phrase, and making the most extraordinary noises when he doesn't know what to say," Ms Thornberry told talkRADIO's Eamonn Holmes.
"Because he is posh he seems to get away with it, but he's not funny now because he's the Prime Minister and we're in really serious times. He doesn't seem to be able to grasp how serious it is."
Her comments came in the wake of Mr Johnson's first-ever Prime Minister's Questions, on what was essentially his second day in the job.
"He was asked some pretty tough questions and just didn't answer them," Ms Thornberry said.
"Remember this Prime Minister was elected, then he had one day and then the whole of the summer off."
During the Commons debate, Mr Johnson repeatedly referred to a Bill which seeks to block a no-deal Brexit as a "surrender Bill".
It is currently being debated by MPs, clearing its first Commons hurdle with a 29-vote majority.
Ms Thornberry continued: "We are not at war with Europe. That is really terrible language to be using. In order to surrender you have to have been at war.
"These are our friends, our neighbours... a lot of Europeans are our partners, our lovers, our bosses, our co-workers."