Top Brexit civil servant warned May's Irish border plan was flawed nine months ago, leaked letter reveals

British plans to avoid a hard border in Northern Ireland created a significant “back door risk” for the European Union, senior civil servants warned as long ago as last August, a leaked letter has revealed.

The warnings of “significant” challenges thrown up by UK plans for a streamlined border were issued last August to Olly Robbins, the UK’s chief Brexit negotiator, in a letter from David Sterling, the head of the Northern Ireland civil service.

Written one week after the UK issued its August 2017 customs paper, Mr Sterling raises concerns about the workability of UK plans which were swiftly dismissed at the time as “magical thinking” by EU negotiators.

Under a sub-heading “Back door risk”, Mr Sterling says that creating the “robust enforcement mechanism” mentioned in the paper would provide a “significant challenge”.

“The example of Turkey illustrates that customs duties are only one part of the picture and technical barriers can be equally disruptive to trade,” he writes.

Mr Sterling’s words would prove prophetic. Last week the Telegraph revealed that senior EU officials had comprehensively rejected the British proposals, citing many of the concerns raised in the letter.

Among these were the practicality of Britain’s demand for an exemption for small and medium-sized enterprises (SMEs) on the border which constitute 80 per cent of cross-border trade - a request that was rejected by the EU.

Mr Sterling warns that since many small businesses - like milk hauliers - contributed to the supply chains of big businesses, the dividing line between large and small enterprises would “undoubtedly be blurred”.

The letter was contained in a tranche of documents published by the hard left GUE/NGL European Parliament group, to which Sinn Fein is affiliated.

The disclosure has sent alarm bells ringing among  Northern Irish businesses. Stephen Kelly, the chief executive of Manufacturing NI, said it betrayed a lack of understanding in Whitehall about the complexity of the border issue.

“It's always been said that a customs union, deal or partnership is helpful but it's not going to resolve the big challenges that exist with the border and David [Sterling] has always made that abundantly clear,” he said.

"We should all be worried that there is not as deep an understanding in London as there is here in Northern Ireland as to what creates a border.

"That said I am reassured that our civil service [in Belfast] are doing what they can, even though they are effectively without an administration," he added, referring to the collapse of Northern Ireland's devolved assembly.

It came as David Davis, the UK’s Brexit secretary, conceded that the so-called “backstop solution” to the Irish border - where Northern Ireland becomes part of the EU’s customs territory - would be a “reserve parachute” if all else failed

Speaking to parliament’s Brexit committee, he also confirmed that the government would return to Brussels and attempt to renegotiate the terms of the UK-EU trade deal if it were rejected by MPs.