Sam Smith wrote the new James Bond theme in just 20 minutes.

The 23-year-old singer was thrilled to be asked to write the official song for 'SPECTRE', the latest movie in the 007 franchise, and said keeping his involvement under wraps was a ''top secret mission of my own''.

Speaking to DJ Nick Grimshaw on BBC Radio 1 about the song, entitled 'Writing's On The Wall', Sam said: ''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.''

Smith co-wrote the track with Grammy Award winner Jimmy Napes and is the first British male solo artist to record the Bond theme since Tom Jones' 'Thunderball' in 1965.

In a statement announcing the song, Sam said: ''This is one of the highlights of my career. 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.''

'Writing's On The Wall' will be released on September 25 while 'SPECTRE' is out in cinemas in the UK on October 26 and in the US on November 6.