A report from the National Audit Office (NAO) has warned that the UK's new customs system may not be ready in time for Brexit.
HM Revenue and Customs is developing a new system for traders wishing to declare customs payments, but the NAO has warned there is a "significant amount of work" to be done, and that it might not be complete by March 2019.
The public spending watchdog has calculated that an extra 180,000 traders will need to start making customs declarations when the UK leaves the EU.
There could be up to 225m declarations a year from 2019 onwards, five times as many as there are currently, the NAO said.
Mike Cherry, national chairman of the Federation of Small Businesses, said: "It's extremely concerning that the UK's new customs system may not be ready in time for Brexit, potentially resulting in massive delays to trade and leaving thousands of businesses in the lurch."
The NAO said the government must decide whether more work is needed to mitigate the risk of HMRC's new system not being up and running in time for Brexit.