LONDON — Labour's Shadow Foreign Secretary Emily Thornberry has urged Theresa May's government to "get a grip" over Brexit and accused cabinet ministers of being in a mess over how to negotiate Britain's exit from the EU.

Speaking during today's Prime Minister's Questions, the Labour MP took aim at senior members of the Conservative front bench over the government's handling of Brexit, including May and Brexit Secretary David Davis.

Thornberry and Tory First Secretary Damian Green were standing in for Labour leader Jeremy Corbyn and Prime Minister May.

That's because May was otherwise engaged, welcoming the King of Spain on a state visit to the UK. When the prime minister is unable to attend PMQs, it is convention that the leader of the opposition does not do so either.

Thornberry, the Labour MP for Islington South and Finsbury asked Green to shed more light on what plans the government has in place in case Britain crashes out of the European Union in March 2019 without a deal in place.

Green refused to disclose the government's plans but reassured the Commons that government ministers expect to walk away from negotiations with their European counterparts with a post-Brexit deal agreed.

"I do appreciate all the First Secretary's answers but they do just serve to illustrate what a mess the government has got itself into by threatening to walk away [from Brexit talks] but talks even began," 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 goes 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 who is so bereft of ideas that she's started putting suggestion boxes around parliament.

"But as a country, we've got 20 months to go until Brexit and we have absolutely got to get a grip and if the party opposite hasn't got the strength for the task then we've absolutely got to get rid of them."

Watch Thornberry attack the government:

"I know he is new to this" @EmilyThornberry "I ask the questions and he answers them - I am quite happy to swap places with them" #pmqs pic.twitter.com/q4Ou5f9zij — DailySunday Politics (@daily_politics) July 12, 2017

Conservative MP Green also confirmed that the Office for Budget Responsibility will on Thursday publish a report outlining the potential economic risks of Britain leaving the European Union.