Unemployment could rise sharply in Northern Ireland if the UK crashes out of the EU, the head of the civil service in the region has said in a letter to local political parties.

Security in the region could also be undermined, David Sterling said, adding: “The planning assumptions include the possibility that, in some scenarios, a no-deal exit could result in additional challenges for the police if the approach appeared to be unfair or unreasonable for some of those most affected.”

Sterling warns that EU tariffs could kill off some farming and food processing interests. “In effect, there is currently no mitigation available for the severe consequences of a no-deal outcome,” the letter says.

“These consequences do not arise from the possibility of checks or controls on either side of the land border, but would simply be the direct consequence of the legal position that would apply. This point is well understood by the business community.”

The letter points out that Northern Ireland, which is one of the weakest of the regions in the UK economically, is particularly vulnerable as 75% of all private sector employers were small- or medium-sized enterprises.

Fears of a no-deal Brexit have grown as the 29 March deadline for leaving the EU approaches and MPs continue to be reluctant to back Theresa May’s withdrawal agreement.

One of the sticking points is the arrangement in the prime minister’s deal for a backstop to prevent the creation of a hard border on the island of Ireland.

The attorney general, Geoffrey Cox, and the Brexit secretary, Stephen Barclay, held fresh talks in Brussels on Tuesday in a renewed effort to secure changes to the backstop to allay fears it could leave the UK trapped in a customs union with the EU.

The foreign secretary, Jeremy Hunt, earlier said the UK was prepared to be flexible over how to address concerns about the backstop and Europe’s leaders were giving “positive signals” on the issue.