Forgery gang arrested in Croatia as police seize more than £500,000 in fake sterling and euro banknotes Police arrested 18 people during raids in five cities around the country Fake euros worth £200,000 and dozens of sheets of forged £50 are seized The £50 sheets were waiting to be cut into individual notes, officials said Fears thousands of notes have already been released onto the market A forgery gang which printed more than £500,000 in fake banknotes has been arrested in Croatia. Police seized forged Euros worth more than £200,000 and dozens of sheets of counterfeit £50 notes and arrested 18 people in raids across five cities and towns in the north of the country. But officials believe thousands of forged bank notes have already been released onto the market throughout Europe. Forgery: Sheets of fake £50 banknotes have been seized by police carrying out raids in Croatia Money, money, money: Euro banknotes worth more than £200,000 were also found during the raids The raids were carried out in Zagreb, Bjelovar, Koprivnica, Varazdin and Cakove. The £50 sheets were waiting to be cut into individual banknotes, according to investigating officers. One official said: 'It is difficult to estimate how many of these notes have already been released onto the market and how long this operation has been going on for. But it was certainly a professional, experienced gang. We arrested 18 people but believe many more to also be involved in this mass organised crime. Police spokesperson Bozo Barbaric said: 'Some of the fake banknotes have already been distributed in other European countries such as Slovenia, Italy and Austria and we think some were already on their way to the UK. It is difficult to estimate how many people are involved in this ring. We had 150 officers working on the investigation. Counterfeit: A machine used to print fake banknotes was found in one of the properties raided by officers The investigation has been going on for months - we have also seized professional printing equipment. Police also confirmed that some of those arrested were previously known to police regarding illegal currency printing and they have been monitoring them for some time. Over the last year over 1,100 fake banknotes have been discovered in Croatia, with south eastern Europe recognised as a hub for the production of fake money in Europe. Dean Savic, anti-organized crime squad director, added: 'In collaboration with Europol we take this organized crime very seriously.' Some 10 million European tourists visit Croatia, which will join the European Union on July 1 this year - during the summer season and therefore fake money could easy be spread. Changing face: Newer £50 notes depict Matthew Boulton and James Watt, from the Industrial Revolution Fighting forgery: The Bank of England uses the latest technology so real notes can be told apart from fakes The seized £50 notes feature the image of Sir John Houblon, the Bank of England's first governor of . But his portrait has replaced on more up to date notes, which depict Matthew Boulton and James Watt, leading figures in the Industrial Revolution. The Bank of England said it took 719,000 counterfeit banknotes out of circulation last year, with a face value of around £13.1million. The Bank of England began rolling out new £50 notes in 2011. The banknotes feature the latest technology so they can be told apart from increasingly sophisticated forgeries. A 'motion thread' on the note is woven into the paper, with five windows along its length that contain images of the "£" symbol and the number '50'. When the note is tilted from side to side, the images move up and down. When the banknote is tilted up and down, the images move from side to side and the 50 and £ symbols switch.