Theresa May must agree a Brexit transition deal "immediately" to end uncertainty for business, Jeremy Corbyn has told the CBI's conference.
The Labour leader backed a call from the business group to get such an agreement by the end of the year.
Mrs May has attempted to reassure business leaders they will get plenty of time to make new arrangements after Britain leaves the EU in March 2019.
She has yet to start talks on her call for a two-year transition period.
But she told the CBI conference in London she wanted the talks to start "as early as possible".
She has previously suggested they can't begin until the UK's sorts out its "future partnership" with the EU, something that might not happen until the middle of next year, if at all.
But some businesses - including BT - have warned that unless the terms of a transitional period were clear by January they would have to start planning for a "hard Brexit".
CBI president Paul Drechsler said about 10% of businesses had "started to reallocate employees, to replan their investments... by the end of the year that will be 25% and by March 60% of all firms will have made big decisions, which are their contingency plans to prepare for the unknown".
That was a message echoed by Mr Corbyn as he told the CBI conference that the transitional arrangements had to be agreed immediately "so that businesses know they won't face a cliff-edge Brexit when the two year negotiating period is up".
"Because let me be clear: to delay a transition deal until a final deal is agreed as the prime minister says she wants to do, is simply not good enough.
"The prospect of sudden changes in the legal and regulatory environment in which people do business is affecting... decisions right now."
He also warned about the "nightmare scenario" of the UK leaving the EU without a trade deal, which he claimed would lead to "tariffs on our food imports and our manufacturing exports, queues at our ports and a hard border in Northern Ireland with all the dangers that could bring".
He added: "The fact that some in the cabinet want 'no deal' to re-launch Britain as a race-to-the-bottom deregulated tax haven on the shores of Europe only adds to the risks."
Last month the CBI and other business groups warned the UK risks losing jobs and investment unless a transition deal is agreed by the end of the year.
But the EU's chief negotiator Michel Barnier has said he will not discuss transition terms until the UK has settled its divorce issues, including its exit bill.
Mrs May told MPs last month the UK could not "put in place the practical changes necessary to move to the future partnership" until it knows "what that future partnership is going to be".
'Crucial to success'
But she used her speech to the CBI to address concerns businesses will face a "cliff edge" in March 2019.
"I have made clear that a strictly time-limited implementation period will be crucial to our future success.
"I know how important it is for business and industry not to face a cliff-edge and to have the time it needs to plan and prepare for the new arrangements.
"During this period our access to one another's markets should continue on current terms, and I want us to agree the detailed arrangements for this period as early as possible.
"But we should also be able to develop our relationships with countries outside the EU in new ways, including through our own trade negotiations."
The CBI director general Carolyn Fairbairn said there was now "cross-party agreement" that a transition deal was needed "as soon as possible" and "so this is the time for urgent action".