According to the central bank, in the first half of 2017, 7,148 banknotes were withdrawn from circulation, 3,588 more than in the second half of 2016.

The 20-euro note is the most likely to be counterfeited, and the central bank withdrew 4,426 fake 20-euro notes from circulation between January and June, followed by 50-euro notes, which accounted for 1,834 of the withdrawn notes.

According to the Bank of Portugal, the seized forgeries are of regular quality and can be identified using the recommended "Touch - Look - Incline" method, and no verification tool is required to spot them.

The banking regulator said the "[seized notes] remain residual when compared to banknotes in circulation."

According to a statement issued by the European Central Bank (ECB), also published on the Bank of Portugal website, in the first half of 2017, some 331,000 counterfeit banknotes were withdrawn from circulation, down from the second half of 2016.

The 20- and 50-euro banknotes accounted for around 85% of forgeries and the majority (96.7%) of counterfeit notes were detected in euro area countries. Around 2.3% of notes were seized in member states Of the European Union and 1% were detected in other parts of the world.

According to the ECB, the probability of receiving a counterfeit euro banknote is now small and so it is a "safe and reliable" method of payment.

"The amount of counterfeiting remains very low compared to the number of genuine euro banknotes in circulation, which has been steadily increasing at rates above gross domestic product (GDP) growth since the introduction of the euro banknotes," the ECB said.

In 2016, the number and value of euro banknotes in circulation increased, respectively, by around 7% and 3.9%.

Currently, it added, more than 20 billion euro banknotes are in circulation, with a total value of over €1.1 billion.