Mexico suspended five members of their Gold Cup squad - Antonio Naelson, Christian Bermudez, Edgar Duenas, Francisco Javier Rodriguez and Guillermo Ochoa - after they all failed a test at a pre-tournament training camp in May.

An investigation by the Mexican Football Association's disciplinary committee found the positive results were a consequence of eating contaminated meat and their decision to hand out no further punishments was backed by FIFA.

WADA challenged the decision, lodging an appeal to the Court of Arbitration for Sport, but today withdrew their objection after reviewing evidence gathered by FIFA.

A WADA statement read: "WADA has subsequently received compelling evidence from a FIFA study at the U17 World Cup in Mexico that indicates a serious health problem in Mexico with regards to meat contaminated with clenbuterol.

"This is a public health issue that is now being addressed urgently by the Mexican Government.

"The government of Mexico - which has legislation forbidding the use of steroids with livestock - accepts that it has an issue with contaminated meat and is actively looking to resolve the problem state by state.

"Already several arrests have been made pursuant to these laws and large amounts of clenbuterol seized. Investigations are to continue.

"WADA applauds FIFA for the further research it has initiated, while WADA, the Mexican Football Federation and the Mexican Government have agreed to assist with the study which will continue as a joint project."

FIFA welcomed WADA's decision in a statement, which read: "This announcement by WADA confirms the initial decision taken by the Mexican Football Association's disciplinary committee to clear the players of any wrongdoing, which FIFA completely agreed with and supported after carefully looking at the case.

"FIFA would also like to thank WADA for positively considering the studies and research conducted by FIFA in this matter, including compelling evidence from a FIFA study at the U-17 World Cup in Mexico that indicates a serious health problem in Mexico with regard to meat contaminated with clenbuterol.

"FIFA is pleased that these studies have served not only to solve this case, but also to assist the Mexican authorities in their fight to tackle this public health issue, which is now being urgently addressed by the Mexican government.

FIFA will continue to play a leading role in this matter and will work together with the various authorities, including WADA, the Mexican Football Association and the Mexican government on this study, which in future will be conducted as a joint project."

WADA added that clenbuterol remains on their list of prohibited substances, and that they would judge further positive tests on a case-by-case basis.