Read a joint statement about this story from the Guardian and Televisa here.

US diplomats raised concerns that the frontrunner in Mexico’s presidential election, Enrique Peña Nieto, was paying for favourable TV coverage as far back as 2009, according to state department cables released by WikiLeaks.

Allegations that coverage by the country’s main television network was biased in favour of Peña Nieto have triggered a wave of student demonstrations in the runup to the election on 1 July. The claims are supported by documents seen by the Guardian, which also implicate other politicians in buying news and entertainment coverage.

One cable, written shortly after US embassy officials were taken on a tour of Mexico State when Peña Nieto was governor, says: “It is widely accepted, for example, that the television monopoly Televisa backs the governor and provides him with an extraordinary amount of airtime and other kinds of coverage.” The document, which dates from September 2009, was titled: “A look at Mexico State, Potemkin village style”.

Another cable from the start of the same year emphasises the importance the then governor Peña Nieto was giving to securing convincing electoral victories for the Institutional Revolutionary party in his state in the upcoming midterm congressional elections that summer.

Peña Nieto, the cable says, “has launched significant public works projects in areas targeted for votes, and analysts and PRI party leaders alike have repeatedly expressed to [US political officers] their belief that he is paying media outlets under the table for favourable news coverage, as well as potentially financing pollsters to sway survey results”.

The cables leaked from the US embassy in Mexico contain frequent mentions of the power that Televisa, and the other main commercial network, TV Azteca, exert over the country’s political elite. The two networks control around 90% of free channels and are widely perceived to be political kingmakers.

This is particularly clear in cables dealing with a new communications law that privileged established interests and was approved by the legislature in the middle of the 2006 election campaign.

One cable dates from February 2006, shortly after the bill was approved by the lower house in just seven minutes with no debate, and before it had been voted on in the upper house.

“With the campaign season in full swing, no one seems to want to upset Televisa or Azteca (which also stands to gain much from the bill) for fear of losing prime advertising slots at good prices.”

The cable surmises that it is “doubtful that any senator will want to risk their future political careers by rocking the boat at a time when all of the parties are deciding their political future”. Similarly, the unnamed diplomat who wrote the cable assumed there was almost no chance that the then-president, Vicente Fox, would veto the law “and risk alienating Televisa”.

Some legislators did make a stand after the bill was approved and a legal challenge was eventually mounted in the supreme court, where the most controversial parts were declared unconstitutional.

In what appeared to be a form of revenge by the political elite on the networks, the newly elected legislature approved an electoral reform in 2007 that banned all paid political propaganda during electoral periods and restricted it outside of them as well.

This, however, was not fulfilling its aim of releasing politics from media pressure, according to one WikiLeaks cable dated June 2009.

“At any rate, parties and candidates are skirting the restrictions,” the cable says. “Journalists and their bosses have been more or less free to engage in the time-honoured Mexican electoral tradition of selling favourable print and broadcast coverage to candidates and parties.”