A key U.K. government customs system has been overwhelmed within weeks of Brexit and threatens to trigger more disruption as freight traffic rebounds.

Exporters say they are struggling to acquire transit documents, which allow goods to enter the European Union without delay, because of a shortage of agents with the authority to issue them.

Agents need to put up a financial guarantee, often backed by a bank, to cover any taxes or duties on the goods being moved — but they have almost all been committed.