Prince George has already hinted he's a die-hard helicopter fan, but according to the Prince William, the four-year-old royal has also taken a particular shine to diggers.

William has revealed how his son was left 'very envious', when he got to drive a digger at the site of a new rehabilitation establishment for injured members of the armed forces, The Telegraph reports.

As patron of the Defence National Rehabilitation Centre, William addressed guests at the City Veterans' Network gala dinner at the Imperial War Museum in London on Thursday evening. During his speech, William recalled visiting the redevelopment project in the Midlands.

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Prince William speaks with guests at the gala dinner

'I have been repeatedly impressed by both the speed and ambitious scale of this endeavour,' he told the audience.

'I was present at the end of 2014 when the first building was demolished to make way for the new construction. George was very envious as I got to drive a digger.'

Speaking about the future of the new centre, which has been in construction on the Stanford Hall Estate since August 2015, William added: 'When completed next year, the DNRC will be a world leading centre for rehabilitation. It will provide pioneering treatments.

George was very envious as I got to drive a digger

'It will give the brave men and women of our armed forces access to the very best rehabilitation facilities on offer, anywhere in the world. You will all agree with me that they deserve nothing less.'

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Prince William, Prince George and Kate Middleton at the Royal International Air Tattoo, July 2016

This isn't the first time the Duke and Duchess have opened up about their son's interests.

In 2015, the Duchess of Cambridge spoke to a Mountain Rescue volunteer during a visit to Wales about how much George loves helicopters.

Pauline Hallett of Ogwen Mountain Rescue said: 'William used to fly us around when he lived and worked up here and I was telling him about the new helicopters,' People reports.

'Catherine then said how much George loved them. He knows all the different colours and asks her about all the parts. She laughed and said she tells George to ask Daddy as she has no idea.'

It sounds like there could be a pilot-in-the-making in the Cambridge family.