The head of Mexico's intelligence service has warned that the country's democratic institutions, including the national Congress, are under threat from powerful drugs cartels.

In one of the frankest admissions yet from a leading authority of the scale of the problem confronting Mexico, Guillermo Valdés, head of Cisen, the government's intelligence organisation, told the Financial Times and a small group of foreign media recently: "Drug traffickers have become the principal threat because they are trying to take over the power of the state."