A secretive unit inside Mexico's predominant television network set up and funded a campaign for Enrique Peña Nieto, who is the favourite to win Sunday's presidential election, according to people familiar with the operation and documents seen by the Guardian.

The new revelations of bias within Televisa, the world's biggest Spanish-language broadcaster, challenge the company's claim to be politically impartial as well as Peña Nieto's insistence that he never had a special relationship with Televisa.

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