Speaking to The Sunday Telegraph 24 hours after a trip to the Jura Mountains in Switzerland, he added: "There were these searing yellow fields and I can’t think of the damn name. I wanted to say something about it but I couldn’t and it wasn’t until we got quite close to Geneva that I thought, of course, oil seed rape."

Sir David also admits that at nearly 91, he does not use the internet or emails, despite working with some of the most pioneering technology in the world to make his documentaries.

In an admission that will no doubt be reassuring for pensioners across Britain, he said: "I’m not a big fan of electronic communication. When it comes to making television programmes, I like to think that I know what the latest gear is and what tomorrow’s latest gear is, but maybe I'm deceiving myself."