UEFA says that an "an organized group of people impersonating genuine stewards" are engaging in the fraudulent scheme, complete with fake bibs and electronic devices to 'check' tickets "as if they were conducting the visual or technical ticket check."

A number of items to support the plot have already been seized by Spanish police but warnings remain that further fans could be targeted in advance of the 8pm (UK time) kick-off in the Wanda Metropolitano Stadium.

As a result, fans are being asked to remain vigilant as to who requests permission to see their ticket.

"Supporters should not show tickets to individuals or small groups of people who could be wearing fake bibs and might approach them on the way to the stadium, outside of the official check zones," a statement by UEFA reads.

Demand for tickets ahead of the fixture is high, with around 34,000 of the 68,000 total tickets being made available to supporters of the finalists, Liverpool and Tottenham Hotspur.

Thousands of supporters who failed to secure a ticket continue to flood into the Spanish capital on Saturday, with some black market tickets reported to have changed hands for as much as £25,000 (€28,2000).

Separate 'fan zones' have been set up in the city for supporters of each team, though these will close just before kick-off because of an official ban on showing the game on a big screen in the city.