The prime minister needs to get a deal approved by Parliament by 29 March to avoid a no-deal Brexit, in which case the UK defaults to World Trade Organization rules.
Labour has accused her of “cynically” running down the clock. It claims Mrs May is planning to delay the final, binding vote on the withdrawal deal she has agreed with the EU until the last possible moment, so that MPs will be faced with a stark choice between her deal and no deal.
The BCC – which represents thousands of firms – says its members are “hugely concerned” that the UK is not prepared for all eventualities.
The business lobby group also warned that the lack of clarity over what will happen had already “stifled investment and growth”.
“There is a very real risk that a lack of clear, actionable information from government will leave firms, their people and their communities hung out to dry,” said BCC director general Adam Marshall.
Mr Marshall said firms remained “in the dark” over crucial issues including contracts and customs tariffs.
“Businesses need answers they can base decisions on, no matter the outcome,” he added.
The BCC has published the list of 20 questions firms want answered. They include whether firms will be able to fly people and goods between the UK and EU after the end of March and whether there will be any import tariffs.