They're calling it "textgate", and it's the biggest voting scandal to hit America since hanging chads and a protracted legal battle turned George W Bush into the nation's 43rd President.

Kris Allen, the latest winner of American Idol, was this week exposed as the unwitting beneficiary of tens, if not hundreds, of thousands of illegal block votes cast at the behest of one of the talent show's high-spending corporate sponsors. AT&T, Idol's official "communications partner", admitted providing free mobile phones and texting services to fans of Allen, a singer, guitar player and pianist, at parties organised in his home town of Jacksonville, Arkansas, on the night of the programme's final episode.

The firm made no similar efforts to support his co-finalist, the eventual runner-up Adam Lambert. Scandalously, its representatives also provided Allen's supporters with lessons in how to send so-called "power texts" - which send 10 or more votes at the touch of a single button. Bobby Kierna, one of the 2,000 guests who attended one of the events, told reporters that she had voted for Allen n a university chum of her daughter n 10,840 times in the AT&T "texting zone" that had been set up there.

Her comments were first published by The Arkansas Democrat-Gazette on Thursday, and rapidly went viral, prompting talk of irregularities in the voting procedure and allegations that AT&T had attempted to "fix" the contest. The phone company issued a swift apology, saying that employees had been "caught up in their enthusiasm", and promising that it wouldn't happen again. But it did little to quell popular outrage or conspiracy theories speculating that Idol's broadcaster, Fox, may have been motivated to influence the show's outcome.

Allen, a clean-cut Christian with a wife and a traditional family background, is a world away from Lambert, a sexually ambiguous singer from Los Angeles who performed the Queen song "Bohemian Rhapsody" in his first audition. In the run-up to the final, their rivalry was widely billed as a political clash between left and right.

In eight years, American Idol, a US version of the British TV show which stars Simon Cowell, has grown into a national institution. It is by some margin the most popular programme on television, at one point boasting 40 million viewers. It has launched the careers of such household names as Kelly Clarkson and Carrie Underwood. But this year, like many network TV shows, it has suffered from falling audiences. The 20 May final attracted just 28.8 million, its lowest rating since 2004. Allen was seen as the most attractive winner in terms of the show's future ability to tap into the lucrative family market.

Talk of an organised effort to rig the final is being fuelled by the refusal of Fox to reveal any details of how scores are counted, or how many votes each finalist polled. All it will say is that roughly 100 million votes were cast in total. In a statement, Fox said it was "absolutely certain that the results of this competition are fair, accurate and verified", claiming an independent monitor was employed to preserve the integrity of the voting process. "In no way did any individuals unfairly influence the outcome of the competition," the statement said.

However, "power texting", together with AT&T's decision to hand out phones at the events, appear to contravene directly Idol's on-screen statement broadcast at the end of each episode warning that votes cast using "technical enhancements" that unfairly influence the outcome of voting can be thrown out.

Even Allen has described the practice as "cheating, apparently", and appeared concerned that the scandal had overshadowed his victory when he appeared on Jimmy Kimmel's late-night chat show this week. "I've no idea what's going on with textgate," he said.