David Davis’s Brexit department suffered a blow as the lead civil servant tasked with finding a solution for the Irish border quit after three months.

Simon Case is leaving his role to become Prince William’s private secretary. He is to be replaced by his deputy, Brendan Threlfall.

As director general Northern Ireland and Ireland in the Department for Exiting the EU, Case led a team of key officials working on what is arguably the most challenging issue facing the department.

Sources said he was leaving for the job in the royal household because it was a “brilliant opportunity”, adding there was no connection between his decision and continuing rows over Ireland’s border.

British officials were in Brussels on Monday for the start of six weeks of talks on Ireland, in anticipation of some resolution on the issue before the next European council summit in June.

A former director of strategy at GCHQ, Case was one of the most senior civil servants working in the UK’s permanent representation office in Brussels before taking up the Ireland job in London. He has also worked as principal private secretary to David Cameron and Theresa May.

Case said it had been a “great privilege” to work on the “vital cross-government effort” on Brexit. He will replace Miguel Head, who has worked with the Duke of Cambridge for 10 years.

“Whilst I will be sorry to leave DExEU, it is a great honour to be have been offered such a unique opportunity to work for His Royal Highness the Duke of Cambridge and I look forward to serving him in the years ahead,” he said.

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