Image copyright PA Image caption David Sterling said cross-border trade could be a "serious dilemma" in the event of a no-deal Brexit

Northern Ireland's chief civil servant has warned a no-deal Brexit could have "grave" consequences for the region.

In a letter to Stormont's political parties, David Sterling comes close to suggesting there may have to be some hardening of the Irish border.

He refers to a "serious dilemma" in finding a solution for trading agri-food products with the Irish Republic.

A no-deal exit could have a "profound and long-lasting impact" on society in Northern Ireland, he added.

Some of the content is similar to a letter Mr Sterling sent to Whitehall departments in December.

But in some areas the language has been toughened including a warning of a "sharp increase in unemployment".

The UK is due to leave the European Union on 29 March. Brexit talks are continuing in Brussels to reach a breakthrough on the backstop.

It is the insurance policy to maintain an open Irish border unless and until another solution is found.

'Material shift'

The most significant part of Mr Sterling's letter is a section dealing with cross-border trade.

He writes that in event of no deal agri-food products from Northern Ireland could only continue to enter the Republic of Ireland if arrangements were put in place to collect tariffs and "fulfil other regulatory obligations".

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If new controls are not in place Mr Sterling said there would be no "legal basis" for this trade.

He said this dilemma could only be resolved by a "material shift in the fundamental position, including the statutory obligations, of one or more of the authorities".

In effect this means that unless the EU waived its usual rules, Northern Ireland produce would not be able to enter the Republic.

The EU normally requires that food products from countries with which it does not have a deal have to enter through a border inspection post.

The letter continues: "In effect, there is currently no mitigation available for the severe consequences of a no-deal outcome.

"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."

'No surprise'

Democratic Unionist Party MP (DUP) Gavin Robinson said the civil service plans for dealing with Brexit "should be taken forward by a functioning" devolved government at Stormont.

The Northern Ireland Executive collapsed in January 2017 amid a bitter split between the DUP and Sinn Féin.

"Sinn Féin walked away and like so many other areas they seem to prefer standing on the sidelines rather than engage in anything positive or productive," said Mr Robinson.

Sinn Féin's deputy leader Michelle O'Neill said the letter outlined how "catastrophic" a no-deal Brexit would be.

And she said the DUP should "start listening" to the civil service and business and farming groups "who are all warning about the disastrous impact".

Aodhán Connolly, the director of the Northern Ireland Retail Consortium, said Mr Sterling's warning should come as "no surprise".

"Let us hope that these facts focus minds as now more than ever we need a deal," he said.

Media playback is unsupported on your device Media caption "Difficult discussions" with the EU and UK if there is a no-deal Brexit, Irish PM says

Mr Sterling's letter comes as the taoiseach (Irish prime minister) announced that the Republic of Ireland would have to have "difficult discussions" with the EU and UK if there was a no-deal Brexit.

Leo Varadkar said his government was continuing its no-deal preparations.

Media playback is unsupported on your device Media caption Confused by Brexit jargon? Reality Check unpacks the basics.

Speaking at leaders' questions, Mr Varadkar said the Irish government had still made no plans for physical infrastructure on the Irish border, under any scenario.

But he added: "If we do end up with no deal in a few weeks' time, we will have to have difficult discussions involving the European Commission and the UK government about how to protect the single market and the customs union, while avoiding the emergence of a hard border on the island."

He said the only "workable" solution so far had been the backstop proposed in the withdrawal agreement and he again dismissed suggestions of alternative arrangements to it.

Meanwhile Mr Varadkar's deputy, Simon Coveney, urged everyone in Ireland to continue to prepare for a no-deal Brexit.

"A clear message to Irish businesses and state agencies is to continue to prepare for no-deal," he said.

"We should not take our foot off the accelerator."