The Confederation of British Industry (CBI) has warned that neither the UK nor the EU is ready for a no-deal Brexit on 31 October.
Its study of all industries, from agri-foods and chemicals, to finance and data, to energy and logistics, has found that they will suffer major disruption and economic pain for "the next decade and more" or more if Oct 31 arrives without an agreement or an extension.
At least 50 trade associations and thousands of companies from all areas of the UK's economy were consulted to deliver 200 recommendations in the report by the CBI.
Businesses are desperate to move beyond Brexit"
The report - titled What Comes Next? The Business Analysis Of No Deal Preparations - added: "Larger companies, particularly those in regulated areas such as financial services, have well-thought-through contingency plans in place, though smaller firms are less well prepared."
In late May, the CBI warned against a no-deal Brexit, with its director-general Carolyn Fairbairn saying it should be an option "that is not even considered".
The group says although businesses have already spent billions on contingency planning for a no-deal exit from Europe, they remain hampered by unclear advice, timelines, cost and complexity.
"While the UK's preparations to date are welcome, the unprecedented nature of Brexit means some aspects cannot be mitigated," said the CBI.
New prime minister Boris Johnson has made Michael Gove responsible for planning a no-deal Brexit. Gove has said the UK government is currently "working on the assumption" of a no-deal Brexit.
He said his team still aimed to come to an agreement with Brussels but, writing in the Sunday Times, he added: "No deal is now a very real prospect."
Josh Hardie, CBI deputy-director general, said: "Businesses are desperate to move beyond Brexit. They have huge belief in the UK and getting a deal will open many doors that have been closed by uncertainty.
"It cannot be beyond the wit of the continent's greatest negotiators to find a way through and agree a deal.
"But until this becomes a reality, all must prepare to leave without one."
A government spokesperson noted that "crucially" the CBI had observed "that the UK is ahead of the EU in planning for no-deal".