The head of Northern Ireland's civil service, David Sterling, has sent a letter to political parties, warning of the potentially "grave" consequences of a no-deal Brexit.

David Sterling predicts that if the UK leaves the European Union without an agreement, it could have "a profound and long-lasting" impact on society in Northern Ireland. 

He suggests there could be a sharp increase in unemployment and that businesses could fail or relocate south of the border. 

He also warns that if a no deal exit occurred, there could be additional challenge for the police if some of those most affected felt they were victims of unfairness or unreasonable treatment. 

The Sterling letter was sent to Northern Ireland's political parties as Westminster MP's, including 10 DUP representatives await the outcome of discussions between the British government and the European Commission in Brussels.

It's expected that the DUP group, led by the party's deputy leader, Nigel Dodds, could have a significant role influencing how the Conservative party responds to what emerges from the talks in Brussels.

In his six page letter, David Sterling draws attention to the vulnerability of Northern Ireland's agri-food sector because of the trading relationship with Ireland and the highly/integrated nature of north/south supply chains. 

He details how the agri-food sector is a disproportionately large large part of the Northern Ireland economy, compared to Great Britain and it is located mainly in rural areas.

He says this sector is particularly vulnerable, given its reliance on cross border supply chain in the production stage and in finished products.

He notes how seventy five per cent of Northern Ireland's private sector employment is in small and medium sized enterprises and that Ireland is the North's largest international export market.