The environment secretary, George Eustice, is facing a threat of legal action from shellfish farmers over claims that the government has misled the industry over its post-Brexit arrangements with the EU. A solicitor representing 20 shellfish firms told the Guardian the government had shown “negligence and maladministration” and that a group action was being considered for compensation. Separately, an exporter of mussels sent a legal letter to the secretary of state saying the firm would sue for damages if the shellfish market with the EU were not opened up by September. The move comes as the UK finally hands a roadmap to Brussels on Northern Ireland following the launch of legal action by the EU over an alleged breach of the withdrawal treaty. Live mussels, cockles, oysters and other shellfish caught in most of the UK’s waters are no longer allowed to enter the EU following Britain leaving the customs union and single market on New Year’s Eve. Eustice, officials and other ministers have claimed the bloc originally planned to let this trade resume after Brexit and that it altered its position earlier this year. Brussels has consistently denied the government’s claims and said the rules for third countries such as the UK are clear and longstanding. Andrew Oliver, a partner at Andrew Jackson LLP, said he was representing 20 shellfish firms considering possible legal action against the Department for the Environment, Food and Rural Affairs: “We are taking a leading counsel’s opinion as to the government’s actions in regard to the EU trade agreement and the assurances given by the government to make live shellfish exports. “We feel that there has been negligence and maladministration regarding the government’s negotiations on the agreement and its treatment of our clients.” One leading mussel farmer has already sent a legal letter to Eustice warning that unless they can begin trading by September they will launch a claim for “substantial damages”. Solicitors for Offshore Shellfish, a 30-year-old business employing 15 people in Brixham, Devon, wrote to the secretary of state on 25 March stating that ministers and officials from the department had repeatedly given false hope to shellfish farmers, suggesting their businesses would be able to continue trading with the EU. “The assurances that were given by the department [Defra] gave rise to a legitimate expectation that export of LBMs [live bivalve molluscs] from class B waters from the UK to the EU would continue after 1 January 2021. “In the event that our clients are unable to restart trade in September 2021, it will become necessary for them to dismantle and remove the offshore farm. This scenario (which we would hope to avoid) may result in a substantial damages claim,” the letter said. The solicitors for Offshore Shellfish said they had received advice from counsel that contradicted Defra’s assurances that exports from the UK to the EU would continue. Their letter said: “Our clients and the Shellfish Association of Great Britain have been in discussions with Victoria Prentis MP (parliamentary undersecretary for farming, fisheries and food) and were informed in a telephone conversation on 9 March that the EU’s position is simply wrong. “However, no legal basis or advice from Defra has been provided to support this position. Indeed, our clients have been forced to seek independent counsel’s opinion on this matter, and counsel advises that the EU position is tenable.” Meanwhile, talks were stepped up on the legal dispute between the EU and the UK over London’s failure to implement the special Brexit arrangements for Northern Ireland. On Tuesday the UK finally delivered the roadmap it had promised last December to Brussels in relation to the implementation of the Northern Ireland protocol. The EU recently launched infringement proceedings against the UK over its decision to extend a grace period for checks on supermarket goods that was due to end on 1 April. It had demanded the UK fulfil its promise of a detailed operational plan setting out the milestones for the implementation along with sharing live border clearance data with Brussels. The European commission said it would study the document in the coming days before commenting. Businesses in Northern Ireland, which feared a further crisis if 1 April passed without progress, welcomed the move. “Looking forward to seeing the detail and how this will work practically for business, but prima facie this is good news. Still a lot to do in little time and our priority remains to keep choice and affordability for NI households,” tweeted Aodhán Connolly, head of Northern Ireland Retail Consortium A Defra spokesperson said the department could not comment on any potential legal action. “The legislation was clear that the export of live bivalve molluscs from class B waters for purification could continue after the transition period. Our correspondence with the commission confirmed this,” he said. “The commission have now amended their import rules, without scientific or technical justification. Effectively, they have changed the law to justify their position in blocking the trade, causing impacts for businesses on both sides.”