Prince William and Kate Middleton have sent a message of support to the people of Alberta, Canada, after the area was hit by severe flooding.

The Duke and Duchess of Cambridge, who visited Canada in 2011, expressed their concern in a heartfelt letter sent to the country’s Prime Minister Stephen Harper.

"Catherine and I have been saddened to learn of the deaths and destruction caused by the unprecedented flooding throughout the Province of Alberta," William wrote.

"Please pass on our best wishes to the Lieutenant-Governor and Premier of Alberta and to the brave emergency services and all those volunteering to help their neighbours during this on-going period of intense efforts.

"Please be assured of our continued thoughts and prayers for all those caught up in the flooding," the Prince added, signing the letter 'William'.

William and Kate, who will welcome their first child in mid-July, hold a special bond with Canada, as it was the first place they visited as a married couple.

In July 2011, just three months after their wedding, the couple arrived in the country to begin a nine-day tour.

Their visit was a huge success, with Prime Minister Stephen Harper remarking, "I'd say we haven’t seen a love-in like that since the first visit of the Beatles."

Kate also revealed on the trip that husband William was helping her settle into her new life. "I'm very, very lucky, he looks after me as much as possible," she told well-wishers.

The couple's message comes days after the region declared a state of emergency as torrential rain led to devastating floods, killing at least three people and displacing more than 100,000 people from their homes.

Soon after receiving their letter, Prime Minister Stephen Harper took to Twitter to thank the royal couple.

"I would like to thank The Duke & Duchess of Cambridge for their prayers & kind words of support," he tweeted. Alberta's Premier Alison Redford also tweeted her gratitude for the message, adding, "Very much appreciate the Duke & Duchess of Cambridge for their support of those affected by #abflood."