A key part of the U.K. government’s post-Brexit border IT system has suffered outages this week, adding to the headache for traders already facing long queues on the way to the country’s busiest port.

The problems have affected the Goods Vehicle Movement Service—which allows companies to file customs forms electronically before they move goods between Britain and the European Union—Her Majesty’s Revenue and Customs said in a statement on Thursday. Firms have been asked to use alternative documentation as part of an effort to “ensure minimal disruption,” HMRC said.

In an update on Friday, the U.K.’s tax authority said the issues had been fixed. Contingency plans will remain in place until midday on Monday and then the full GVMS requirements will be fully restored, it said.

The system failures have added a layer of complication for companies shuttling goods across the English Channel. They had already been facing 20-mile-long lines of trucks on the approach to Dover after P&O ferries suspended sailings. 

Without GVMS, firms have had to use a different set of forms to get their goods through customs, according to Shane Brennan, chief executive of the Cold Chain Federation, which represents businesses operating frozen and chilled storage distribution vehicles.

Read More: U.K. Trade and Travel Face More Disruption at Key Channel Port

“It’s just reinforcing for people—particularly the European-based haulage operations—the hassle of dealing with the U.K.,” he said. “It’s going to be a real problem if it carries on and compounds our ongoing problem of having enough capacity to get stuff into the U.K.”

Haulers said the timing couldn’t be worse, with Easter being one of their busiest periods of the year. P&O has said it is preparing to resume some services from this weekend, a move which may ease the crunch at the border.

“Easter is already really tight for capacity,” said Jon Swallow, co-founder of Felixstowe-based Jordon Freight, which moves goods between Britain and the European Union. The GVMS outage “just adds to the problem,” he said.