According to Open Britain, a hard Brexit could spark an “avalanche of paperwork” if thousands of importing and exporting businesses were forced to fill out similar forms to those required in moving goods and services beyond the union.
The claim comes as the government faces another potential legal challenge to its Brexit plans, this time from two campaigners who are seeking a judicial review in the high court to keep Britain in the single market.
Under the EU customs union, which deals with rules relating to trade, unified tariffs are levied on imports to the whole area, meaning no new charges or associated paperwork are needed for goods shipped between members.
According to Open Britain, which has support from Remain MPs in the three main Westminster parties, customs data shows that last year the UK made 70.5mn import declarations and 6.5mn export declarations over trade in non-EU goods.
The group says that if leaving the customs union involved a similar level of customs paperwork, it would require businesses to complete more than 45mn import declarations and 15mn export declarations every year.
This contradicts the arguments of pro-Brexit campaigners made before the June referendum, namely that leaving the EU would save businesses from EU-related bureaucracy, said the Labour MP Chris Leslie, who backs the group.
“Brexit isn’t a liberation from red tape but the beginning of an avalanche of paperwork for businesses trading with Europe,” he said. “Ministers should commit to publishing a full cost-benefit analysis of the consequences of leaving the customs union, rather than just airily asserting that it will benefit our trade and our economy without any evidence for those claims.”
Jo Carberry, of Open Britain, said leaving the customs union would result in “a bombshell of Brexit bureaucracy for British businesses”.
Meanwhile, membership of the single market, the EU’s wider agreement on free trade as well as the movement of people, services and money, could now be argued in the high court, the Sunday Times has reported.
As the Supreme Court considers the Article 50 legal challenge, a new case hinges on whether the government would have to trigger another legal measure to quit the single market.
Claimants in the new case argue that the government has no mandate to withdraw from the single market because it was not on the ballot paper on June 23 and was not included in the Conservative party manifesto for last year’s general election. They argue that the government would have to trigger Article 127 of the European Economic Area agreement.
However, the former attorney general Dominic Grieve – one of Open Britain’s supporters in parliament – cast doubts on the legal basis for the new challenge.
“It’s possible that the argument is that the way we leave the single market has to be separate from leaving the EU itself,” he said, speaking on Sky News’ Murnaghan programme on Sunday. “I have some doubts as to whether that would work.”
Asked if the Article 50 challenge was simply “the first legal spanner” in the government’s plans and other legal challenges would follow, Grieve said there was “undoubtedly a very large number of legal complexities”. He added: “It’s going to be complicated; it’s 50 years of enmeshed legal obligations and rights.”
The new case comes after it emerged that another legal challenge to Brexit will go ahead in Ireland after crowdfunding hit a £75,000 target within 48 hours. Specialist tax barrister Jolyon Maugham said he was bringing the case to address two issues: whether a notification under Article 50 can be revoked and whether Britain would automatically leave the EEA when it left the EU.
Maugham argues that the European council and the European commission may have breached EU law on the grounds that if Article 50 cannot be revoked then the government will be forced to accept whatever Brexit deal the EU offers. As an EU nation needs to be named in the legal action, the case will be heard in Ireland’s high court but is likely to be referred to European courts.
The single market court challenge is being brought by Remain voter Peter Wilding, chairman of pressure group British Influence, and Adrian Yalland, a Conservative lobbyist who voted to leave.