The Liberal Democrats will claim to be “the real party of business”, because the hard Brexit plans of both Theresa May and Jeremy Corbyn would wreck the economy.

Vince Cable, the party’s treasury spokesman, will attempt to reignite its troubled campaign by warning both Labour and the Conservatives are poised to “inflict maximum economic damage”.

He will point out that both the major parties have now rejected membership of the EU single market and customs union – threatening a dramatic slump in trade, according to economic experts.

Business leaders are desperate to stay in the customs arrangement, to allow exporters to sell into the single market without the huge inconvenience of having to fill in forms or go through customs checks.

Meanwhile, “the economic storm clouds are already gathering”, Dr Cable will warn, amid rising inflation, falling real wages, rising personal debt and vanishing growth.

He will say: “If we crash out of the single market and customs union and revert to World Trade Organisation terms, respected independent estimates suggest that our trade will slump by almost a third by 2030.

“Far from turning Britain into a centre for exports, the main thing we would export under Theresa May would be jobs.”

Dr Cable will warn that leaving the EU with “no deal” would see tax receipts from the financial sector alone crash by £10bn a year and cost 75,000 jobs.

“This would devastate the economy, but Theresa May just doesn’t seem to care,” he will allege, speaking in London.

And, turning on Labour, the former Business Secretary will attack the party for being equally guilty in having “turned their backs on business and the economy”.

He will say: “The May-Farage extreme Brexit that Labour voted for will drive out high-earners and leading international companies, leaving lower tax receipts for public spending.”

The speech comes as the Lib Dems flatline in the polls, after making a second Brexit referendum the key plank of their election manifesto – threatening party leader Tim Farron with a loss of seats.

However, the party has been buoyed by an endorsement by The Economist magazine, which criticised “a backward-looking Labour Party and an inward-looking Tory party about to compound its historic mistake over Brexit”.

Dr Cable will add: “We are also the only party that genuinely recognises the crucial role of entrepreneurship in generating job creation, innovation and productivity improvements that are fundamental to our economic success.

“As our endorsement by The Economist shows, we are rapidly becoming the party of business.”

The “economic storm clouds” include, the veteran former MP will say, inflation at a four-year high of 2.7 per cent, while UK real wages fall for the first time in three years.

GDP growth in the UK slowed to 0.2 per cent in the first quarter of 2017, the weakest quarterly growth in 12 months.