Europe’s busiest ferry port warned U.K. businesses it needs to do more to prepare for possible delivery delays post-Brexit, as it emerged one in three haven’t made plans to cope with customs changes when the U.K. leaves the European Union next year.

A survey from the Port of Dover and British Chambers of Commerce published Monday found that while more than a third of traders relied on the so-called just-in-time delivery of goods, 33 percent of all businesses still haven’t made plans for possible changes to checks and declarations between the U.K. and EU. The survey consulted 835 businesses across the U.K.

“For the sake of U.K. plc, it is vital that fluidity at Dover and throughout the supply chain is maintained,” said Richard Christian, head of policy and communications at the Port of Dover. “There is no substitutable capacity elsewhere that can handle the type and volume of goods.”

The government has said Britain will leave the EU’s customs union, which allows goods to travel unencumbered through the bloc—despite pleadings from lawmakers across parties for Prime Minister Theresa May to change her mind. The House of Lords voted in favor of staying in the customs union this month and, while not binding, it could embolden rebels in May’s party to vote the same way in the House of Commons as soon as next month.

The Port of Dover handles as much as 122 billion pounds ($168 billion), or 17 percent, of the U.K.’s trade in goods. Nearly a third of companies believe they will be impacted in terms of administration, costs or operations by delays or congestion at U.K. or European ports after Brexit.

“Firms need to know what checks and declarations they will have to go through on trade with the EU – and need to start planning for the changes ahead,” said Adam Marshall, director general of the BCC. “The lack of certainty over the terms of the final settlement with the EU makes it hard to plan with confidence, but businesses shouldn’t be complacent.”