The European Central Bank is putting a new €20 banknote with enhanced security features to foil counterfeiters into circulation. 

A new "portrait window" near the top of the hologram stripe becomes transparent when seen against light and reveals a portrait of Europa, a figure from Greek mythology. 

"Cash was, is and will continue to be an important means of payment. The introduction of the new €20 banknote is tangible proof of this," said ECB Executive Board member Yves Mersch at the ECB's headquarters in Frankfurt today. 

The new notes, 4.3 billion of which have been produced so far, will gradually replace the €20 banknotes of the first series, which will however remain legal tender. 

Like the newer €5 and €10 bills, the new €20 note has an emerald-coloured number that changes hue from green to blue depending on the angle it is viewed from. 

The ECB launched the €5 note in May 2013 and the €10 note in September 2014. 

The new €50 bill is set to go into circulation in the second quarter of 2017, reported the Bank of France in a separate press conference