"At no point since the Second World War has Britain's ruling elite so recklessly put the country in such an exposed position without any plan," the Labour leader told supporters in Peterborough, central England.
"As a result they're now reduced to repeating 'Brexit means Brexit'. They're unfit to negotiate it."
Corbyn, who campaigned for Britain to stay in the EU, said he accepted the result of the June referendum and would not block May's plans to begin exit negotiations by the end of March.
"There can be no question of giving Theresa May's Tories a free pass in the Brexit negotiations to entrench and instill further the failed free market policies in post-Brexit Britain," he said.
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May has yet to outline her negotiating strategy, but has made clear she wants to end unrestricted immigration from the rest of the EU -- a key element of single market membership.
Corbyn is a strong advocate of free movement of labour, but some of his MPs fear this position is alienating voters concerned about the impact of mass migration.
He accused the government of blaming migrants for overstretched public services, which he argued were the result of six years of austerity.
Labour has been languishing in the polls for months. The latest YouGov survey put them on 26 per cent, compared to 39 per cent for May's ruling Conservatives.