Ministers have "absolutely got to get a grip" on Brexit and reveal the impact of a "no deal" scenario, Emily Thornberry has said.
The shadow foreign secretary mocked Theresa May and her Cabinet, claiming the Prime Minister is "bereft of ideas" and "putting suggestion boxes around Parliament" while Foreign Secretary Boris Johnson is "making it up as he's going along".
Ms Thornberry also likened Anne Marie Morris to former BNP leader Nick Griffin after the MP was suspended from the Tory parliamentary party for describing the prospect of leaving the EU with no deal as "the real n***** in the woodpile".
Ms Thornberry deputised for Labour leader Jeremy Corbyn at Prime Minister's Questions as Mrs May was absent due to her involvement in the King of Spain's state visit.
This resulted in First Secretary of State Damian Green answering for the Government and claiming Labour has had "nine different plans" on Europe, which has led to splits among the Opposition.
Exchanges during PMQs were noticeably punchier between the two stand-ins, with Ms Thornberry seeking to make fun of the Cabinet ministers and their leadership ambitions.
Ms Thornberry said she believed there were "plenty of other people on the frontbench who would love the opportunity to audition as prime minister" if Mr Green did not want to continue in the role.
She added the Government was in a "mess" after threatening to walk away before Brexit talks began.
Ms Thornberry said: "Isn't the truth now that we've got a no deal option on the table but they won't tell us what that means?
"They've got contingency plans but won't let the public see them.
"We've got a Chancellor demanding transitional arrangements, which a no deal option makes impossible.
"We've got a Foreign Secretary making it up as he's going along.
"We've got a Brexit Secretary so used to overruling his colleagues that he's started overruling himself.
"And we've got a Prime Minister so bereft of ideas that she's started putting suggestion boxes around Parliament.
"But as a country we have got 20 months to go until Brexit.
"We have absolutely got to get a grip.
"If the party opposite hasn't got the strength for the task, we've absolutely got to get rid of them."
Mr Green said negotiations are "going well", with the Government wanting to protect the rights of EU citizens living in the UK and Britons living in the member states as soon as possible.
He said: "That is the sign of a practical, pragmatic Government getting on with work in the interests of the British people.
"What we would have, as we've seen, from the Labour Party is they've so far - I've counted - had nine different plans on Europe.
"They want to be both in and out of the single market, in and out of the customs union.
"They said they wanted to remain, they voted for Article 50 - they split their party on that."
Mr Green also pointed to the General Election result, claiming his party won.
Ms Thornberry opened by noting Mr Green was the 16th Tory MP to speak for his party at PMQs since he was elected in 1997.
Mr Green, in his reply, prompted cheers from Tory MPs when he said: "There are many, many distinguished people of both sexes who have done it in this party as we, of course, elect women leaders."
Ms Thornberry countered by saying only three Tory women have taken on the role at PMQs in the 20-year period, adding she believes three female Labour MPs have done it in the last two years.
She repeatedly pressed Mr Green about whether "no deal" was still an option for the UK Government as part of Brexit talks.
The Labour frontbencher asked: "Are ministers just making it up as they're going along or is it still the Government's clear policy that no deal is an option?"
Mr Green said it is conceivable the UK would be offered a "punishment deal", noting this would be worse than no deal.
He added: "It's not our intention, we want to have a deal, we want to have a good deal."
Mr Green said Labour would accept any deal, saying this was a "terrible" approach to a negotiation.
Ms Thornberry joked Mr Green had failed to read the PM's memo, telling the Commons: "You're supposed to be building consensus, man.
"If we ignore the political bluster, I think what we heard was that no deal is indeed still an option.
"If that is the case, can we turn to what I might call the East India Club question?
"Because before (Newton Abbot MP Ms Morris) suddenly turned herself in to Nick Griffin this was the question she was trying to ask - what does no deal actually mean for our businesses, for our people and for issues such as the Irish land border?"
Shouts of "shame" could be heard in relation to the remark about Ms Morris.
Ms Thornberry later criticised the Government's "confusion" over whether a contingency plan for the possibility of no Brexit deal exists.
After raising comments from Mr Johnson and Downing Street, Ms Thornberry said: "The Brexit Secretary (David Davis) might be laughing but I'm turning to him next."
Ms Thornberry began laughing before adding: "The Brexit Secretary was so busy fighting with himself that on March 12 he said there was a plan, on March 17 he said there wasn't, on May 19 he said he spent half his time thinking about it, and yesterday he said he wasn't prepared to comment."
Mr Green, in his reply, said: "The (Office for Budget Responsibility) is publishing its fiscal risks report tomorrow, so if she can be patient she will see the report she wants."