Image copyright Chris Jackson / Getty Images Image caption Prince George Alexander Louis was born on 22 July 2013

An official portrait of Prince George has been released to mark his fourth birthday.

The picture, taken at Kensington Palace ahead of his birthday on Saturday, captures a smiling future king.

The Duke and Duchess of Cambridge were "delighted" to share the photograph taken by royal photographer Chris Jackson, Kensington Palace said.

The prince has spent the run-up to his birthday on a tour of Poland and Germany with his parents.

Prince George Alexander Louis - known as His Royal Highness Prince George of Cambridge - was born on 22 July 2013.

"The Duke and Duchess are very pleased to share this lovely picture as they celebrate Prince George's fourth birthday, and would like to thank everyone for all of the kind messages they have received," Kensington Palace said.

Getty Images royal photographer Mr Jackson, who took the photo at the end of June, said: "I'm thrilled and honoured that the Duke and Duchess of Cambridge have chosen to release this portrait to celebrate Prince George's fourth birthday.

"He is such a happy little boy and certainly injects some fun into a photoshoot."

Image copyright PA Image caption The prince spent five days in Poland and Germany with his parents ahead of his birthday

Earlier, the Duke of Cambridge gave Prince George and Princess Charlotte a guided tour of a helicopter at the Airbus factory in Hamburg on the last day of their official tour of Germany and Poland.

Prince George tried on a pilot's helmet while Princess Charlotte played with buttons in the cockpit.

In September, Prince George is due to start school.

He will go to Thomas's Battersea, a private preparatory school located a few miles from the family residence in Kensington Palace in London, where the family will be based.