At least £1m worth of seafood a day will be stopped from leaving Scotland this week as delays continue at UK ports, according to an industry body.
Scotland Food and Drink said Brexit red tape and IT issues were continuing to cause huge problems.
Its comments came as exporters reported further delays in moving goods to EU countries.
The UK government said it was providing "guidance and support" to firms affected.
Seafood logistics firm DFDS Scotland warned it would take until at least Wednesday to resolve problems.
Last Thursday, the company suspended export groupage - truckloads combining multiple small consignments - after facing paperwork and IT difficulties.
DFDS said "significant progress" had since been made in resolving the problems, but it would take several days for normal service to be resumed.
In a statement, it said: "Rectifying problems from the first couple of days has taken considerable time and effort and added to the delays but more robust processes are now falling into place.
"The backlog is greatly reduced but every step of the customs procedure is taking longer than anticipated and capacity is consequently reduced.
"Despite our extraordinary efforts it is clear we need to further suspend the Groupage Export Service, which includes smaller consignments of fish and shellfish, until Wednesday at the earliest."
'EU door closed'
James Withers, from Scotland Food and Drink, said most of the main haulage firms had now effectively closed the door to the EU market for exporters of perishable products, including seafood and red meat.
He said: "Now, if you are exporting seafood, for example, you have got about 18 different steps to go through, and the person who is importing from the EU it has got another eight to work through just to get that product."
Mr Withers said there were "a lot of the problems here in our own backyard as well", as haulage firms in Scotland struggled with bureaucracy.
Mr Withers added: "Most of the main haulage firms are stopping groupage, which is basically the consolidation of different products within the same load.
"I reckon there's probably at least £1m a day of seafood alone that will not be able to leave Scotland over the next few days because this form of exporting, this groupage, has halted.
"It might get started on Wednesday, but I think that sounds optimistic."
Shaun Prentice, who is trying to ship agricultural and forestry machinery from Scotland to Ireland, said his company had been hit by red tape.
He told BBC Radio's Good Morning Scotland: "We seem to be having problems mostly with getting paperwork and the codes required to try and ship any of the lorries from the UK across to Ireland.
"We tried to ship a couple of lorries last week from Holyhead across to Dublin.
"These lorries still haven't managed to ship across from there, and there's been other lorries sitting down there for four of five days trying to get the correct paperwork."
A UK government spokesman said: "Businesses and hauliers have made huge strides to get ready but we were always clear that there would be some disruption at the end of the transition period.
"Although many businesses have moved goods successfully since 1 January, we are aware of some issues, and are providing guidance and support.
"Hauliers can use our 'Check a HGV is ready' service to make sure they have correct paperwork, and must obtain a Kent Access Permit if they plan to travel via Dover or Eurotunnel."