It comes after the Bank of England withdrew 300,000 counterfeit notes out of a stock of more than 2 billion last year.

With 200 million in circulation, the £50 note is the least common in Britain, compared to 1.6 billion £20 notes, but it is instantly recognisable.

The new £50 note is the first to carry two portraits on the reverse – entrepreneur Matthew Boulton (1728-1809) and engineer James Watt (1736-1819), who produced steam engines in Britain and abroad.

It replaces a portrait of Sir John Houblon, the first Governor of the Bank of England.

For Bank Governor Sir Mervyn King, Boulton and Watt are perfect role models.

“The partnership of an innovator and an entrepreneur created exactly the kind of commercial success that we will need in this country as we rebalance our economy over the years ahead,” he says.

The new £50, launched by The Bank of England’s chief cashier, Chris Salmon, whose signature also features, includes a so-called ‘motion thread’ woven into the paper rather than printed on it.

The security feature has five windows, running from top to bottom, each containing images of the £ symbol and the number 50.

When the note is tilted from side to side, the images move up and down; when it is tilted up and down, the images move from side to side.

“We want people to be aware there is a new £50 note, to be aware of the new security feature,” says Salmon.

“We are very efficient at detecting counterfeits as well so our sense is that once they enter circulation they are removed very quickly,” the chief cashier claims.

The existing £50 was launched in 1994 and is the oldest of the current notes in circulation.

It succeeded the Wren note, which was introduced in 1981. Prior to this, there was a white £50 note in issue between 1725 and 1943.

Salmon is responsible for everything to do with banknotes, from production and design to distribution and circulation.

He says, “one of my jobs next year will be to decide which is the next note to be replaced, what security features to put on it, and what characters to put on it”.

He dismisses the idea of another denomination, such as a £100 note, on top of the £5, £10, £20 and £50 notes in circulation.

“Our view is the current four denominations are fit for purpose.  I don’t think there will be any change there.

“But it’s something you would never say never to. It’s one of our jobs to keep an eye on it.”

The Bank of England has issued banknotes since it was founded in 1694, but it has not printed money on Threadneedle Street since 1920. Printing has taken place at the Bank’s site in Essex since 1956. It is currently done by British firm De La Rue.

The Bank has always signed its notes, first as a ‘promise to pay’ to someone depositing gold and now as a mark of trust.

The organisation has offered some tips so you can identify authentic notes:

1. Check the paper and the raised print. Banknotes are printed on special paper that gives them their unique feel. By running your finger across the note, you can feel raised print in areas such as the words ‘Bank of England’ on the front.

2. Check the metallic thread. There is a metallic thread embedded in every banknote. This appears as silver dashes on the back of £5, £10 and £20 notes and on the front of £50 notes. If you hold the note up to the light, the metallic thread appears as a continuous dark line.

3. Check the print quality. The printed lines and colours on banknotes are sharp, clear and free from smudges or blurred edges.

4. Check the hologram. There is a hologram on the foil patch. If you tilt the note, the image will change between a brightly coloured picture of Britannia and the numerical value of the note.

5. Check with ultra-violet light. If you put the note under a good quality ultra-violet light, its value appears in bright red and green numbers while the background is dull in contrast.

6. Check the microlettering. Using a magnifying glass, look closely at the lettering beneath the Queen’s portrait – you will see the value of the note written in small letters and numerals.