A special unit of Mexico's dominant television broadcaster has worked since at least 2009 to advance the presidential candidacy of Enrique Peña Nieto, who is widely favored to win the July 1 contest, Britain's Guardian newspaper said Wednesday.

Documents show that a team inside Televisa paid for and distributed videos praising Peña Nieto and his Institutional Revolutionary Party, or PRI, ahead of the 2009 midterm legislative elections, the daily said.

The PRI's 2009 success in the central state of Mexico - where Peña Nieto was then governor - solidified his status as the party's presumptive 2012 presidential nominee.

The new revelations about Televisa, the world's largest Spanish-language media conglomerate, cast doubt on the network's claims to impartiality, the Guardian said in a front-page story.

Known as "project Handcock," the unit sought to operate below the radar and team members were told not to use Televisa e-mail addresses or IPs when disseminating the partisan political videos, a source told the British paper.

The nature of Peña Nieto's relationship with Televisa became an issue in the election campaign with the emergence last month of a movement of university students focused on media favoritism toward the PRI candidate.

One of the people identified by the Guardian as a director of project Handcock, Alejandra Lagunes, is now part of the PRI campaign team.

Televisa declined the Guardian's request for comment on the latest allegations, which follow a story three weeks ago in the same newspaper that cited 2005 documents showing the network collected payments from Peña Nieto and others for favorable coverage.

Those documents also revealed a Televisa policy of attacking Andres Manuel Lopez Obrador, the leftist who barely lost the 2006 presidential election.

The PRI governed Mexico without interruption from 1929-2000, a regime once described by Nobel Prize-winning novelist Mario Vargas Llosa as "the perfect dictatorship."

That era ended with the election in 2000 of the conservative National Action Party's Vicente Fox, who was succeeded six years later by party colleague Felipe Calderon after the closest contest in Mexican history.

But the PRI has benefited politically by high levels of drug-related violence in recent years and appears poised to take back the presidency with the telegenic Peña Nieto as its standard-bearer. EFE