The New Zealand trade deal is another "nail in the coffin" of British mussels farmers, who have warned they face being forced out of business by a combination of cheap imports and Brexit.
Boris Johnson's decision to slash tariffs on New Zealand mussels from up to 20 percent to zero over four years was described as "another slap in the face" to an industry devastated by the loss of its major EU market after Brexit.
"The principle of crucifying our industry to the benefit of imports is a crazy argument," said David Jarrad of the Shellfish Association of Great Britain.
"The New Zealand import issue may not put producers out of business by itself but it is another nail in the lid of a coffin that is already closing," Mr Jarrad said.
EU exports became impossible after Brexit took effect on January 1 because the UK was treated as a non-EU country under Brussels' public health rules.
That meant live bivalve molluscs must be purified before exporting to the EU. When the UK was a member state, live mussels would be transported to EU purification facilities but there are no UK equivalents. British efforts to secure a lighter touch regime from Brussels have so far failed.
James Wilson, who runs Bangor mussel company Deepdock, which exported 95 percent of its product to the EU, blamed government "arrogance and ignorance".
"Our ability to access our customers in the EU is heavily curtailed and subject to huge ongoing uncertainty," he said, "It is crazy given that we were good at what we did, very science based and produced a good mussel that was well regarded in the major consuming markets like Belgium and France."
The UK Mussel industry produces about 15,000-25000 tonnes of product a year, with British consumption of live and partially cooked mussels at just 6,000 tonnes.
The most common type of mussel in UK waters in Mytilus edulis, the blue mussel. The New Zealand mussels is the slightly different Perna canaliculus or the green lipped mussel.
While the majority of British mussels are exported live, New Zealand mussels are exported frozen, which makes them popular in the catering industry because they are more convenient.
"If the outcome of the new trade deal is that there is easier and cheaper access to the UK market for New Zealand mussels, then invariably there will be some kind of negative effect on domestically produced mussels," Mr Wilson said.
The New Zealand mussels industry has grown from humble beginnings in the early 1980s to producing about 100,000 tonnes of product each year, worth about £130m at first sale value. UK mussels exports to New Zealand already face no tariffs.
"This deal will see a gradual removal of tariffs over three years for Kiwi mussels, giving the UK industry time to adjust, with mechanisms also in place to provide a safety net if firms face serious harm from increased imports," a government spokesman said.
"We continue to support our fantastic seafood sector, and earlier this year we committed £23m to help producers with impacts from leaving the EU and Covid-19."
Trade expert Sam Lowe, of the Centre for European Reform think tank, said, "The real problem isn't so much the additional competition from some islands on the other side of the world, rather that it is happening at the same time as them being closed out of selling to their biggest export market."