The head of Northern Ireland’s civil service has privately warned Downing Street that the UK’s plan for the Irish border fails to solve the major problems caused by Brexit.

In a leaked private letter to Theresa May‘s Brexit chief seen by The Independent, David Sterling said the plans in the UK’s position paper on the Irish border suffered from a number of practical problems and did not address certain “issues”.

He also raised concerns that Whitehall was not engaging closely enough with local NI officials with a good understanding of issues like the Good Friday Agreement and North-South cooperation.

The customs proposals, which the UK government has still been trying to sell to EU negotiators Brussels as recently as last week, were first laid out in an official position paper on Northern Ireland and customs published in August last year.

Days after the paper was released Mr Sterling wrote to Olly Robbins, at the time permanent secretary of the Brexit department, to voice his concerns about its content. Downing Street, where Mr Robbins now heads up the Brexit unit, however confirmed last week that UK negotiators are still arguing for the solutions proposed in the position papers in talks, stating that “the two proposals we have put forward remain the basis for our negotiation position”.

The same proposals interrogated by Mr Sterling in the letter were reportedly rubbished by EU officials during the talks and subjected to a “systematic and forensic annihilation” during meetings with the European Commission in Brussels.

The letter to Mr Robbins, dated 23 August 2017 says: “While the position papers have helpfully started a dialogue on movement of goods, many of the issues raised in June still require exploration.”

While the position papers have helpfully started a dialogue on movement of goods, many of the issues raised in June still require exploration David Sterling, head of the Northern Ireland civil service

In the letter the top official in Northern Ireland also appears to be frustrated by a lack of involvement of local officials in drawing up the policy, stating that “we would like to see a more intensive and open engagement between Whitehall and NICS [Northern Ireland Civil Service] officials”. He goes on to list seven areas in which Whitehall had not been taking full advantage of local knowledge, including on the Good Friday Agreement, North-South cooperation, and trade.

Going into detail about the issues with the UK customs proposals, Mr Sterling said plans to exempt small businesses from customs suffered from issues of practicality.

“The dividing line between businesses of different sizes in Northern Ireland supply chains will undoubtedly be blurred. This could make it difficult to accurately define which businesses are exempt from customs processes,” he said.

“For example, bulk tankers owned or contracted by Irish milk processing firms collect raw milk from multiple farms in Northern Ireland before returning to Irish processing facilities.”

He adds: “Small businesses trading on the island of Ireland may also be using inputs from outside these islands, or be part of an island of Ireland supply chain that ultimately sells into world markets.”

The NI executive civil service chief also highlighted concerns with plans for a “robust enforcement mechanism” to keep goods from crossing the border.

“The visibility and impact of a ‘robust enforcement mechanism’ to ensure goods not complying with the EU’s trade policy (whether through a lower tariff or diverging standard) stay in the UK is a significant challenge,” he said.

“The example of Turkey illustrates that customs duties are only one part of the picture and technical barriers can be equally disruptive to trade. Potential trade barriers created by diverging tariffs or regulatory standards are therefore a key area for ongoing discussion. DAERA is currently engaged with DEFRA on complex sanitary and phytosanitary (SPS) issues. “

In a separate leaked document also seen by The Independent, an internal economic analysis drawn up by the Northern Ireland executive’s Department for the Economy warns that the province’s economy will suffer because of Brexit whatever solution is applied.

“There is no quick or simple solution to the land border,” the assessment, dated May 2017 says. “There is likely to be some negative impact on the economy whether border controls are sited at the land or sea border, at least in the short-to-medium term. The policy question is therefore how these impacts can be minimised and what transitional support is required.”

The Government has declined to comment on the leaks.

There has still be little progress on the border question in talks this year, despite the opening of talks about the future relationship last week.

Under the Good Friday Agreement there can be no hard border between the Republic of Ireland and Northern Ireland, and both sides agree that none should return.

The EU however says the customs union and single market must have external border checks to maintain their integrity. Commission negotiators effectively proposed moving the customs checks to the Irish Sea between Great Britain and Northern Ireland, but Theresa May refused the approach after pressure from her allies in the DUP.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

The draft withdrawal agreement as envisaged by the EU says Britain can come up with a specific solution to the border problem, remove the need for a hard border by negotiating a close trade relationship with the EU, or revert to a “backstop” option that would keep Britain in full alignment with relevant EU regulations.

But with Ms May having ruled out membership of the single market or a customs union the chance of solving the problem looks slim, barring a U-turn.

Speaking in the European Parliament in Brussels on Wednesday Guy Verhofstadt, the body’s Brexit coordinator, said he believed a solution to the border problem was needed by the end of June. UK Brexit secretary David Davis said earlier this week that he was aiming for October.