The Duke and Duchess of Cambridge have signed a virtual visitors' book - with their first joint tweet.

William and Kate's foray into the world of social media had a twist as they used old technology - in the form of a morse code telegraph machine - to send their virtual message.

Their brief words - effectively an electronic signature - were transmitted to the world during a visit to a Canadian museum telling the story of the Yukon, famed as the home of the Klondike gold rush.

Radio operator Doug Bell tapped out the Cambridges' tweet when their tour of the visitor attraction in Whitehorse - known as the Wilderness City - brought them to the settlement's old telegraph office, now a preserved exhibit.

As Mr Bell, who last sent a telegram in 1947, was sending the dots and dashes, William joked: ''Ah, there's a spelling mistake.''

The royal couple both held the lever down to send the tweet, posted under the Twitter account Telegraph to Tweet, which said: ''The Duke and Duchess of Cambridge, September 2016, Whitehorse Yukon.''

The telegraph machine operated by Mr Bell, a former commissioner of Yukon, was wired up to a micro control unit, which converted the electrical signal into letters and was connected to a PC.

Mr Bell went on to tell the couple about the time he chatted to William's parents, the Prince and Princess of Wales, when they visited Ottawa in 1981.

The 90-year-old said: ''I met your parents'' and the Duke replied: ''It seems pretty much everyone has met my family here.''

Mr Bell added: ''I asked your father if he'd been to Yukon and he said no, and I said we ought to do something about that so I called the governor general and we made arrangements. Then I got a call saying 'They can't go, the princess is pregnant'.''

''Wow, that must have been me or Harry,'' said William.

Kate, who wore a red Carolina Herrera coat, was impressed with the telegraph office, visited by the Queen in 1959 and one of the few surviving historic buildings in Whitehorse.

As the Duchess looked around the building, she said: ''Wow, that's amazing."