Uefa says “an organised group of people impersonating genuine stewards” is behind a plot to steal tickets for the Champions League final.
Spanish police seized fake steward bibs and fake devices for the technical ticket check, as well as fake accreditation in Madrid.
Uefa says the group’s aim was to steal tickets “as if they were conducting the visual or technical ticket check”.
Liverpool face Tottenham in the all-English showpiece at 20:00 BST.
Fans are asked to only show their tickets at clearly marked check zones outside the Wanda Metropolitano.
“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 Uefa statement said.
Spurs and Liverpool fans will share 33,226 tickets for the final, which takes place at the 68,000-capacity home of Atletico Madrid, with hundreds more expected to flood the city and soak up the atmosphere.
Liverpool have won the European Cup or Champions League five times, most recently in 2005. They have twice been finalists since then – beaten in 2007 by AC Milan and in 2018 by Real Madrid.
Tottenham had never previously qualified for the final of the top European club competition and will hope to end their 11-year wait for a trophy.