King Willem-Alexander tells of worries about trade and says he is backing Michel Barnier

It has been condemned by experts and divided the country. Now Brexit has been officially given the royal seal of disapproval – albeit by a Dutch monarch from his palace in The Hague.

Ahead of the first state visit by a Dutch royal to the UK in almost 40 years, King Willem-Alexander of the Netherlands has told of his regret and fears over Britain’s decision to leave the EU.

It might not be an intervention that will convince leave voters that Brussels is not a project of the elite for the elite, but the king, speaking to British journalists at Noordeinde Palace in The Hague, said he expected to see an impact on trade between the two nations.

He also warned that despite his love of the British sense of humour, Monty Python, and the witty best man speeches he had heard at weddings in the UK’s most exclusive country estates, his royal household was squarely behind Michel Barnier, as the EU’s chief negotiator prepares to engage in the final months of debate with Downing Street over the future trading relationship and the Irish border.

Willem-Alexander, who cannot be quoted directly under the protocols of the Dutch royal household, said he had yet to see any evidence of success by the British government in its attempts to go over the head of the EU negotiator to get a better deal from member states.

The bastion of Barnier had yet to be breached, he told reporters in his palace’s Putti room, in which the walls are decorated by wingless angels and the fireplace is gilded in gold.

Willem-Alexander also insisted he admired Britain’s liberal attitude – and its airports, Inverness and Belfast in particular. The king until recently co-piloted commercial KLM flights twice a month, often to the UK.

He was speaking as Buckingham Palace confirmed that the king and his wife, Queen Máxima, would make a state visit to Britain on 23 and 24 October, as the two countries seek to build ties in preparation for Brexit.

Dutch palace officials said the visit would cement links between the two countries as “North Sea neighbours” with shared values in the past, present and future.

Brexit is a focus of the trip, with the king and queen set to meet Dutch citizens who are concerned about their business prospects in the UK.

A Dutch palace official said: “They will talk with Dutch entrepreneurs, academics and creatives, volunteers from the Dutch community, and Dutch nationals who are concerned about the consequences of Brexit.”