One of the most dangerous drug cartels in the world has infiltrated the US Embassy in Mexico and America's top anti-trafficking agency, it emerged last night.

A captured informant codenamed Felipe admitted to Mexican prosecutors that he used his job as an Interpol agent working at the US Embassy in Mexico City and at the international airport in the city to feed classified information about anti-drug operations to the feared Beltrán-Leyva cartel.

The revelation came as prosecutors also admitted that two staff in the Mexican Attorney-General's Office for Organised Crime - a government unit that fights the drug mafia — had been found to have been in the pockets of the cartel for four years, as were at least three federal policemen with inside information on surveillance targets and potential raids. Each were paid between $150,000 (£97,000) and $450,000 a month by the cartel.

It was the worst known case of law enforcement in Mexico being compromised by drug lords since the arrest in 1997 of General Jesús Gutiérrez Rebollo, the head of the country's anti-drug agency, who was convicted of assisting Amado Carrillo, a kingpin.

“This doesn't say much for US security — it's as embarrassing as hell for this to come out and I suspect heads will roll within the DEA [Drug Enforcement Administration],” said Bruce Bagley, an expert in Latin American drug trafficking, from the University of Miami in Florida.

The scandal came five days after Condoleezza Rice, the US Secretary of State, travelled to Mexico City to discuss the $400 million Mérida Initiative, a package to help Mexican and Central American law enforcement agencies to fight organised drug crime. “We've already achieved an outstanding level of co-operation in our efforts to fight drug trafficking and organised crime... the United States considers this important initiative and its implementation an urgent task,” she said.

President Calderón had appealed for the money, which will help to fund resources such as helicopters and surveillance aircraft, to be speeded up as Mexico struggles against a surge in drug violence.

The money will be filtered through the US Embassy in Mexico City.

The Beltrán-Leyva cartel is an offshoot of the Sinaloa cartel, the largest Mexican drug trafficking confederation, and has engaged in a bloody turf war as it strives for supremacy.

“It's a very big deal that they got inside the embassy and the DEA. The Beltrán-Leyva cartel has got a serious leg-up by doing this and while we don't know how much information they got, in this war knowledge is power,” Dr Bagley said. “These guys are an emerging player and they have to be taken seriously.”

Despite the corruption Mexico continues to capture top smugglers. On Saturday Eduardo Arellano Félix was arrested after a shootout in Tijuana. He had allegedly been running the Arellano Félix cartel with his sister.

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