Sector leaders told the International Trade Select Committee the cost of disruption hads already caused some businesses to shut their doors for good.
Ian Wright, chief executive of the Food and Drink Federation (FDF), told MPs on Thursday food exports have been cut by at least half since the start of January despite the deal.
He told the committee: “We agree with other trade groups that food exports to the EU have declined by 50 to 60 per cent in January.
“That may be because companies have stockpiled three or four months of goods on the other side of the Channel and they may bounce back, but that is a big number to recover in the next few months.”
He said he was “particularly concerned” about uncertainty among officials overseeing new trade checks.
“This is not trivialising it, but in many cases they are making it up as they go along, because they don’t know what would happen with particular certificates,” he said.
Mr Wright also warned that the UK could be “50,000 customs agents” short of what is needed when import regulations are enforced from April.
Marine and fisheries consultant Terri Portmann told MPs that seafood businesses are already shutting amid the impact of Brexit disruption.
“It has been an unmitigated disaster,” she said on Thursday.
“We have already seen seafood businesses who heavily relied upon an export market close their doors – companies that have been around for 30 to 40 years.
“And I suspect there are many more that are currently hanging on by their fingernails and going bankrupt slowly, because part of the problem is that with fresh seafood products, you can’t stockpile.”
Additional reporting by Press Association