In a statement, law enforcement said the seizure was the equivalent of 20 million fentanyl doses and would cause an economic hit to organised crime of an estimated US$400 million.

The record seizure comes amid worsening violence in Sinaloa where factions of the Sinaloa Cartel have been engaged in bitter fighting, which flared after the capture of kingpin Ismael “El Mayo” Zambada in July.

It also comes as US president-elect Donald Trump has vowed to get Mexico to do more to stop the flow of the deadly opioid that has killed hundreds of thousands of Americans, threatening to impose steep tariffs if he does not see results on curbing fentanyl trafficking and illegal migration.

“This is an investigation that has been going on for a long time and yesterday, it gave these results. It is the largest seizure of fentanyl pills ever made,” President Claudia Sheinbaum told a press conference on Wednesday.

Security forces found the fentanyl at two properties in the municipality of Ahome, which intelligence work and tip-offs from the public had led them to investigate.

In one building, law enforcement found 800kg of fentanyl, some precursor chemicals and four vehicles. In the other, they discovered 11 packages totalling about 300kg of fentanyl, as well as precursors, scales and industrial mixers.

The announcement marks a change in tack for the Mexican government with former president Andres Manuel Lopez Obrador repeatedly denying the country was a centre for the production of fentanyl despite significant evidence showing that it was.