A man has been threatened with jail after warning tourists about a dodgy currency exchange business in the heart of Prague.

Social media user Janek Rubes today posted a guide to Prague’s “worst tourist trap” to YouTube .

Video shows Mr Rubes introducing Cheque Point, located near the city’s iconic Astronomical Clock, as a “horrible” and dishonest exchange rate shop which has been operating for “decades”.

He claims the business offers tourists 15 Czech Republic Koruna (CZK) for one Euro (EUR), instead of the official rate of 27 CZK per Euro.

Mr Rubes implores unassuming tourists to “ask before you change”, and offers them a map to a “better tourist exchange place”.

After turning away dozens of visitors over two hours, state police arrived to shut down Mr Rubes’ mission and tell him it is against the law to hand out flyers.

“I can do this. I will do this. Because I’m trying to save people from losing money,” he proclaimed in the video.

Spotting a legal loop-hole, Mr Rubes showed tourists the flyer, and continued to spread his message.

He was eventually served legal papers from a Cheque Point representative who claimed Mr Rubes had cost the business “hundreds of thousands of Czech dollars” and was warned he could face jail for up to two years.

The video ends with Mr Rubes urging viewers to “share this message… let people know this is an epic tourist trap”.