Liberal Democrat leader Sir Vince Cable is accusing ministers of perpetrating a "fraud" on voters with their claim Britain can look forward to growing trade links outside the EU.
In his keynote speech to the party's spring conference in Southport on Sunday, Sir Vince will say Brexit will mean "tearing up" dozens of preferential trade deals Britain enjoys as an EU member.
He will say the Government is guilty of "utter naivety" in putting their faith in "the Wild West warmonger in Washington and the bully of Beijing" - Donald Trump and Chinese President Xi Jinping - rather than the UK's established trading partners in Europe.
At the same time he will launch a renewed attack on Jeremy Corbyn, saying the Labour leader continued to work with Theresa May "to make Brexit happen" while pursuing a policy of "doctrinaire socialism" in the UK .
He will appeal to Remainers across the political spectrum to rally behind the Lib Dems' demand for a referendum on the final Brexit deal.
Amid a looming trade war following Mr Trump's decision to impose tariffs on steel and aluminium imports, Sir Vince will say the Government's vision of "a Global Britain signing new trade deals hither and thither" is a "fraud".
"Far from opening our arms to the world, we will be tearing up preferential trade deals we already have with 27 countries in the EU and 74 outside it," he will say.
"There is no more eloquent testimony to the Government's utter naivety about trade, that at a time when the world is descending into trade war, they put more faith in the Wild West warmonger in Washington and the bully of Beijing than they do in our established friends and trade partners in Europe."
On Labour, Sir Vince will say that while the party had made a "few tentative steps towards sanity" it was still "strongly committed" to co-operating with the Conservatives to ensure Brexit goes through.
In doing so, he will say, Mr Corbyn is letting down the people he claims to be defending.
"You cannot speak up for the poor and be complicit in making the country poorer," he will say.
"You cannot be an advocate of strong rights at work, and stand by while your country walks away from the organisation which has most stood up for workers.
"They believe you can be for Brexit and against it. The time has come to decide - doctrinaire socialism in one country or open, practical co-operation with the biggest single market in the world."