Brexit should be delayed if no trade deal can be struck with the EU by the end of the two-year negotiating process, business leaders have said.
The British Chambers of Commerce (BCC) also wants businesses to continue to be allowed to recruit skilled and low-skilled EU workers after the UK leaves.
The business organisation is holding its annual conference on Tuesday.
Theresa May has said she will trigger Article 50 by the end of March which would start the process of leaving.
The BCC said completing a trade deal within the two years allowed by Article 50 would be the "ideal outcome".
But it continued: "Should this prove impossible, we should seek an extension to the negotiating period to enable completion of both agreements concurrently."
However, such an extension would only be possible if all EU member states agree.
Business Secretary Greg Clark and shadow chancellor John McDonnell will be among those addressing the conference in London.
BCC director general Adam Marshall said: "Business communities across the UK want practical considerations, not ideology or politics, at the heart of the government's approach to Brexit negotiations.
"What's debated in Westminster often isn't what matters for most businesses.
"Most firms care little about the exact process for triggering Article 50, but they care a lot about an unexpected VAT hit to their cash flow, sudden changes to regulation, the inability to recruit the right people for the job, or if their products are stopped by customs authorities at the border."
BBC business correspondent Jonty Bloom
It's on immigration that the BCC is most likely to be disappointed.
It's calling for its members to be able to recruit across the EU after Brexit with minimal bureaucracy, costs or barriers.
But it's difficult to see how that is compatible with the government's stated aim of bringing immigration down to tens of thousands a year.
Or how it could square with the prime minister's recent declaration that Brexit must mean control of the number of people who come to Britain from Europe.
The BCC's comments come after former Prime Minister Sir John Major warned that the chances of no agreement being reached within the time limit were "very high".
In a speech, he warned that an "unreal and over-optimistic" vision of Brexit was being put forward.
Downing Street said the government was determined to make a success of Brexit.