Charles took William and Harry litter picking during school holidays

The Duke of Cambridge has revealed that his father, the Prince of Wales, lives "the way he advocates" when it comes to his environmental beliefs and took him "litter picking" during a school holiday.

William and his brother, the Duke of Sussex, thought it was normal to be given spikes and black plastic bags as teenagers and encouraged to clear up other people's rubbish.

Charles is well known for his campaigning to protect the rain forests, save endangered species from extinction and combat the threat of climate change.

William and Harry recall the memory in a new hour-long BBC documentary marking Charles's forthcoming 70th birthday.

Speaking in Prince, Son And Heir: Charles At 70, William said: "He took us litter picking, when we were younger, on holiday.

"We were in Norfolk on school holidays, and went out litter picking with him ... Both of us thought 'This is perfectly normal, everyone must do it'.

"We're there with our, basically, spikes, stabbing the rubbish into black plastic bags."

The comments were made as the royal brothers watched a recording of the prince giving a speech in 1970 in which he mentioned the growing throwaway plastic society.

William went on to say that his father was affected by the backlash he received for his environmental views, which saw him ridiculed for supposedly talking to plants.

He said: "He does live the way he advocates so, until someone comes up with an electric plane – it's impossible to get round the world, for instance, without using a plane that's there.

"He did take to heart the criticism quite a lot when he was younger."

Harry confessed he was "programmed" to collect litter because of the positive influence of his father.

He said: "He's done an amazing job without telling us what we should be doing or what direction we should be going.

"He's just let us learn from the nature of the job, learn from him, learn from Mummy, to the point where I used to get the ... mickey (taken) out of me at school for just picking up rubbish."

Prince, Son And Heir: Charles At 70 will be broadcast on Thursday at 9pm on BBC One.