The Bank of England has been showing off a new security feature which it hopes will protect new bank notes from counterfeiters.
A thread containing moving images is being used on the new £50 note as it enters circulation.
The bank said the green "motion thread" has five windows featuring the pound symbol and the number 50, which move up and down when the note is tilted from side to side.
The thread has been woven into the new-style note, which celebrates the 18th century business partnership of entrepreneur Matthew Boulton and engineer James Watt.
The Boulton and Watt note is the first from the Bank of England to feature the motion thread. It is also the first time two portraits have appeared together on the back of a Bank of England bank note.
The £50 is the first note to be signed by Chris Salmon, who was appointed the Bank's chief cashier in April.
He told Sky News that of the UK's 2.8 billion bank notes, only 300,000 were found to be fakes in 2010 and taken out of the public domain.
Mr Salmon said the £50 note was the oldest in circulation and therefore next in line for a makeover.
"The Boulton and Watt £50 banknote has new and enhanced security features which demonstrate the Bank's commitment to its role of maintaining public confidence in the currency," he said.
"The motion thread security feature is one of the new measures which should help the public identify genuine £50 banknotes."
The new note will initially circulate alongside the current £50 note which features Sir John Houblon, the first Bank of England governor.