Mexican officials have admitted that they had advance warning that the United States authorities were planning a huge investigation into the laundering of drugs money.

The operation led to the arrest last month of around one-hundred-and-sixty people, including officials at leading Mexican banks.

The Mexican authorities protested, saying American undercover agents had violated Mexican national sovereignty.

The attorney-general's office in Mexico City now says that US customs officers did inform them about the planned investigation in 1996, but at no point were American agents given permission to operate in Mexico.