New £50 note: Why Alan Turing is on it and when do old banknotes expire?

The new £50 note featuring mathematician, code-breaker and computer scientist Alan Turing launches today - the final bank note to switch from paper to plastic.

It will replace the note featuring steam engine pioneers James Watt and Matthew Boulton, who first appeared on the note in 2011. 

The launch of the new £50 note follows the successful introduction of a polymer £5 note in 2016 featuring Sir Winston Churchill, £10 note featuring Jane Austen in 2017, and £20 last year with painter JMW Turner.

When is the new £50 note coming out?

The bank note enters into circulation today, June 23 - the mathematician’s birthday.

Why is Alan Turing on the new note?

Mr Turing was pivotal to cracking the German codes enciphered with their Enigma machine during the Second World War. This allowed the allies to read German naval communications and helped shorten the war. 

In a nod to Mr Turing's code-breaking skills, GCHQ has released 12 puzzles that the agency's chief puzzler says will take seven hours to crack. 

Mr Turing also played a pivotal role in the development of early computers and set the foundations for work on artificial intelligence by considering the question of whether machines could think. 

Turing was homosexual and was posthumously pardoned by the Queen, having been convicted of gross indecency for his relationship with a man. 

Alan Turing's portrait is based on a photo taken in 1951 by Elliott & Fry which is part of the Photographs Collection at the National Portrait Gallery.

When is the old £50 note expiry date?

The Bank of England will announce the withdrawal date of the old note after it has issued its new plastic £50 note.

Even after this date, it will still be possible to deposit withdrawn notes into bank accounts, directly or via the Post Office. The Bank of England will also exchange withdrawn notes for new ones for all time.

How to tell your £50 note is genuine?

There are two key security features to help confirm the authenticity of your note: the hologram featured on the note changes between ‘Fifty’ and ‘Pounds’ when the note is tilted and by checking the see-through window on the note.

The metallic image over the main window will have gold and green foil on the front and silver on the back. The image on the front foil squares will change between '50' and a '£' symbol when the note is tilted.

Other security checks include looking for a second, smaller window in the bottom corner of the note; a portrait of the Queen is printed on the window with a '£50 Bank of England' printed twice around the edge; there is a silver foil patch containing a 3D image of the coronation crown is above the main see-through window on the front of the note.

Further ways of checking include making sure the metallic, red foil patch contains the letters 'AT@ on the back of the note, as well as feeling for the raised print on the polymer.