Denmark would need to offer Greenland to the UK first before the island could be sold to Donald Trump, it has emerged.

An agreement dating back to 1917 means Britain has first refusal on the Arctic island if current owners Denmark decide to sell it, according to Tom Høyem, Copenhagen’s representative in Greenland from 1982 to 1987.

President Trump has said he wants the US to acquire the Danish autonomous territory, which is rich in rare minerals. He reportedly threatened to impose tariffs if Denmark refuses to sell it, in a bad-tempered phone call with Danish prime minister Mette Frederiksen.

However, Denmark insists the 836,300-square-mile island is not for sale.

Mr Høyem, an expert on Greenland’s tangled history, said promises were made to the UK when the US tried to buy the island once before, during the presidency of Woodrow Wilson.

He said: “If Trump tried to buy Greenland, he would have to ask London first. The United Kingdom demanded in 1917 that if Greenland were to be sold then the UK should have the first right to buy it.”

The UK was interested in the island because Canada, at that time a British dominion, is just a few miles away across the sea. What’s more, a tiny island called Hans in the Nares Strait is split between Canada and Greenland, which means they technically share a land border.

It’s thought unlikely the UK would ever attempt to take advantage of the arrangement.

Mr Høyem also said President Wilson signed a document confirming that Greenland “is and will forever be Danish”.

He told the Sunday Times: “This means the United States has legally accepted Greenland is and will always be Danish. But Trump, it seems, has never heard that.”

Mr Trump has predicted America will gain control of the island, telling journalists: “I think the people want to be with us.

“I don’t really know what claim Denmark has to it, but it would be a very unfriendly act if they didn’t allow that to happen because it’s for the protection of the free world.

“I think Greenland we’ll get because it has to do with freedom of the world.

“It has nothing to do with the United States other than that we’re the one that can provide the freedom. They can’t.”