The Government has produced no evidence to support Theresa May’s assertion that “no deal is better than a bad deal” in the Brexit negotiations, a new Commons report has concluded.

Given the admission by the Brexit Secretary David Davis last month that the Government has carried out no economic assessment of crashing out of the EU without a deal, MPs on the Exiting the European Committee said the Prime Minister’s threat “is unsubstantiated”.

The 155-page report adds that officials should set out what contingency planning is taking place and urges the Government to undertake an economic and legal assessment of leaving the EU without a deal.

It says that in these circumstances, Parliament must be given a vote on whether to accept Ms May’s decision to leave without a deal at the end of negotiations. It comes just days after the Prime Minister served notice of Article 50 to EU leaders, triggering the two-year countdown of Britain’s exit from the union.

But the report risked being undermined after several Brexit-backing MPs on the 21-member cross-party committee were reported to have walked out of a meeting last week, protesting the reports conclusions as “too gloomy”.

Five Conservatives – including former ministers John Whittingdale and Dominic Raab – and Democratic Unionist Sammy Wilson voted against the report, but were outnumbered by 10 Labour, Tory, Liberal Democrat, SNP and SDLP committee members, all of whom backed Remain in last year’s referendum.

Mr Raab told the Press Association: “The report was rushed, skewed and partisan. After two reports that had strong support, it's regrettable that this one split the committee. That undermines its credibility and influence, but I hope and expect the committee will learn the right lessons as we move forward.”

Commenting on the report, the committee’s chair Hilary Benn, Labour MP and former Shadow Foreign Secretary, said Britain was about to enter into “enormously important and complex negotiations covering trade, customs rules, access to the single market, security and foreign policy cooperation and the rights of UK and EU citizens at home and abroad”.

Mr Benn continued: “Without an economic impact assessment of ‘no deal’ and without evidence that steps are being taken to mitigate the damaging effect of such an outcome, the Government’s assertion that ‘no deal is better than a bad deal’ is unsubstantiated. Parliament must be in an informed position to decide whether a proposed deal is, in fact, better or worse than no deal.”

“Leaving the EU without a future trade deal and in doing so defaulting to World Trade Organisation (WTO) rules is no less an important decision for the UK’s economic future than the terms of any future Free Trade Agreement between the UK and the EU. It is therefore essential that such a step is not taken without Parliament having a vote on the matter."

Giving evidence to MPs last month, Mr Davis insisted it was not possible to calculate the impact of the negotiations failing, adding: “I may be able to do so in about a year’s time.