Police have seized an array of concoctions used to conjure up the cocktail in Barcelona that contained traces of E.coli, which is a bacteria from faecal matter.
When ingested, it can cause gastroenteritis and in some cases, death.
Members of the Nature Protection Service (Seprona) of the Civil Guard of Barcelona and the Beaches Unit of the Urban Guard of Barcelona found the cocktail products discarded in public sewers near beaches.
Benito Granados, of Barcelona police, said: “The added value of this operation is that we now have the scientific confirmation that these products have a bacterium that can be dangerous.”
Mr Granados confirmed evidence of the bacterium in the products following laboratory tests.
He also said E.coli had been found in bags containing ice and mint, a common feature of Mojitos.
Humberto Quiroga, Barcelona’s chief lieutenant, added: “They stored the products directly inside containers or sewers.”
Among other ingredients seized there was also a green powder, with an unknown composition and could be used to give flavour and colour to the drink.
All of them were sent to the Department of Chemistry and Environment of the Criminalistics Service of the Civil Guard for the analysis and preparation of the relevant reports.
The results confirmed that these substances can be harmful due to the presence of E.coli bacteria.
The revelation prompted greater control measures to take place in Barcelona, a popular holiday hotspot for British tourists.
Police have cracked down on the sale of the drinks from unlicensed vendors and have also identified 18 suspects involved in the preparation of such cocktails.
These were from Romania, Pakistan and India, police confirmed.
The news follows the shocking deaths of John and Susan Cooper, from Burnley, Lancashire, who died in an Egypt hotel where high levels of E.coli were uncovered.
The couple died on August 21 while staying at the Steigenberger Aqua Magic Hotel in Hurghada.
Tests on the food and hygiene standards at the hotel identified a high level of E.coli and staphylococcus bacteria, Thomas Cook said.
However, the cause of the couple’s deaths is still unknown, according to independent specialists who carried out the investigation.
Additional reporting by Maria Ortega.