Only half of UK firms that trade internationally have considered the impact of Brexit on their business, says the British Chambers of Commerce.
It said only 52% of them had carried out a risk assessment ahead of the end of the Brexit transition period on 1 January 2021.
The BCC said firms needed more clarity.
It added that it had written to Cabinet Office Minister Michael Gove seeking action for businesses and urgent discussions to help firms prepare.
Exporters to the EU face 7,000-truck-long queues in Kent and two-day delays to trade after the Brexit transition period ends, according to a letter sent to the freight industry by Mr Gove.
On Wednesday, Mr Gove told Parliament that it was up to firms to get ready for the end of the transition period.
"The consequences of a lack of business preparedness will be not just economic opportunities missed for those companies who don't prepare, but potentially much wider disruption," he said.
The BCC's director general, Adam Marshall, said businesses still had "significant unanswered questions" and called on the government to "ramp up engagement with business urgently".
The BCC also complained of a lack of information with which to plan, while companies were suffering from "deadline fatigue".
'There will be a few sleepless nights'
As Andrew Lawless sees the end of the Brexit transition period approaching, he has found himself having second thoughts about his vote in the 2016 referendum.
The sales director of Clitheroe-based packaging firm Phoenix Handling Solutions and member of the East Lancashire Chamber of Commerce, voted in favour of Brexit, because he thought it would be good for his business. His fellow directors did the same.
"But when we realised that the trade agreements were a bit of a sticking point, we did actually wonder if we'd made the right decision," he told the BBC.
"We're only a small SME and we import virtually all our equipment. We don't want to let our blue-chip customers down because of delays at the ports."
The company's customers include Coca-Cola, Pepsi, Nestle and Heineken.
It now sees itself having to take on paperwork that was previously handled by its suppliers in Italy, the Netherlands and France.
"To be honest, we're a bit apprehensive, because we're not 100% certain we're doing the right things," he says. "I'm sure there will be a few sleepless nights."
"With just 98 days to go, business communities face the triple threat of a resurgent coronavirus, receding government support schemes and a disorderly end to the transition period," said Mr Marshall.
"Despite recent public information campaigns, base levels of preparedness are low. Many firms say they've heard talk of deadlines and cliff-edges before, and others are still grappling with fundamental challenges as a result of the pandemic."
The BCC said companies were still in the dark on a number of issues, including:
- Firms do not know what rules of origin will apply after the transition period, preventing them and their customers from planning and potentially creating unprecedented new administration and costs
- There is no clarity on how food and drink due to be sold in the EU and Northern Ireland is to be labelled
- There is very limited guidance on the movement of goods from Britain to Northern Ireland.
The BCC's data came from a survey conducted between 17 and 21 August and is based on responses from 527 firms.