A hard border between Northern Ireland and the Irish Republic would create particular problems for the agriculture and food sectors.
Food and live animals account for the largest share of trade with Ireland. Northern Ireland is reliant on the republic for more than 60 per cent of its food and live animal exports. Agri-foods are particularly important to Northern Ireland and the sector is 'one of the few economic bright spots' according to the CBI.
Aiden Gough of InterTradeIsland told the Financial Times that 'The food industry is absolutely predominant in the cross-border trade in the island. The vast majority of trade is supply chain and goods cross the border multiple times before coming final products.'
A quarter of Northern Ireland milk and more than one-third of its lamb are processed at plants across the border. Baileys liqueur is produced in Ireland and sent north for bottling before returning to the republic for export.
Shaun Murphy at KPMG says that agriculture is 'the sector that is most at risk' because 'integrated cross-border supply chains are complex and costly to unravel'.
Given that the UK Government is not prepared to countenance Northern Ireland staying in the customs union or internal market, it is apparently prepared to consider continued regulatory convergence between the north and south of Ireland to prevent border problems.
However, that solution is unacceptable to the DUP whom the Government depends on for its majority. It would in effect create a border in the Irish Sea. It might also attract objections from other member states who could portray it as giving an unfair advantage to Northern Ireland.