Sam Smith's James Bond theme is the quickest song he's ever written.

The 23-year-old singer has recorded the theme song for upcoming James Bond movie 'Spectre' and has lifted the lid on the process that saw him handed the honour.

Reflecting on the song, entitled 'Writing's On The Wall', he told BBC Radio 1: ''I'm so relieved to actually talk about this, this has been a top secret mission of my own.

''I got called into the office with (producer) Barbara Broccoli and (director) Sam Mendes and they gave me the script, I read the script ... they said, 'Have a go at the song.'

''It's the quickest I've ever written a song - it took 20 minutes - and they loved it. I love the song so much.''

Meanwhile, Sam admitted the Bond theme song is ''one of the highlights of my career''.

He added: ''I am honoured to finally announce that I will be singing the next Bond theme song.

''I am so excited to be a part of this iconic British legacy and join an incredible line up of some of my biggest musical inspirations. I hope you all enjoy the song as much as I enjoyed making it.''