When do I need to exchange old £20 notes?

The new polymer £20 note will replace the paper £20 note featuring economist Adam Smith and has been hailed by the Bank of England as its most secure banknote yet.

It includes two see-through windows and a two colour foil to help thwart counterfeiters.

The Bank expects half of all ATMs across the UK to be dispensing polymer £20 banknotes in just two weeks' time.

The son of a barber and a wig maker, Joseph Mallord William Turner (1775-1851) became renowned as one of the great masters of painting.

He first exhibited at the Royal Academy in 1790 aged just 15.

He became known as "the painter of light" and had a keen interest in depicting nature such as extreme weather conditions and the violent power of the sea.

(The note features Turner's self portrait. Picture: PA)

Features on the new Turner note include

- A large see-through window with a blue and gold foil on the front depicting Margate lighthouse and Turner Contemporary. The foil is silver on the back. The shape of the large window is based on the shape of the fountains in Trafalgar Square.

- Turner's self-portrait, painted around 1799 and currently on display in Tate Britain.

- One of Turner's most celebrated paintings The Fighting Temeraire - a tribute to the ship HMS Temeraire which played a distinguished role in Nelson's victory at the Battle of Trafalgar in 1805. It was voted the nation's favourite painting in a BBC Radio 4 poll.

- A metallic hologram which changes between the word "twenty" and "pounds" when the note is tilted.

- A purple foil patch containing the letter "T" and based on the staircase at Tate Britain.

- A quote "Light is therefore colour" from an 1818 lecture by Turner referring to the use of light, shade, colour and tone in his pictures.

- Turner's signature from his will, in which he bequeathed many of his paintings to the nation.

Can I still use my £20 notes?

The paper £20 notes can still be used as normal and the Bank will give six months' notice ahead of their legal tender status being withdrawn.

There are over two billion £20 notes in circulation.

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(Governor of the Bank of England Mark Carney holds the note aloft. Picture: PA)

How can I exchange my £20 notes?

You won't need to exchange them until the Bank gives six months' notice.

The simplest and quickest way to exchange old notes is to deposit them with your bank.