X Factor viewers have complained about the introduction of a sing-off in last night's semi-final, which allowed the judges to have the final say over who was kicked off the programme.

In previous series the final stages of the competition have been decided solely by public vote, but yesterday Cher Lloyd and Mary Byrne were given the chance to fight for survival with a sing-off.

The decision of Simon Cowell, Dannii Minogue and Cheryl Cole to save Lloyd over the Tesco checkout worker sparked anger on the DS forums and Twitter, where fans complained that the judges' vote had been introduced deliberately to save Lloyd.

On the Saturday live show host Dermot O'Leary implied that it would be the viewers deciding who went home, stating: "The vote is now open, and will close in tomorrow night's result show. Now it's all in your hands. It's your votes and your votes alone that can get your act safely through to the final."

There were also listings on the official X Factor website, which claimed that "the act with the fewest votes" would leave the show, and Louis Walsh stated in a radio interview last week that it would be down to the public vote.

An X Factor spokesperson denied any allegations of a fix, claiming that the plans for a semi-final sing-off came into place a few weeks ago. The show rep told DS: "As the show evolves there are always changes to format. This decision was made weeks ago."

A show source added: "This series has seen double eliminations, wildcards and four finalists, as well as the introduction of an audience to auditions last year. In seven years the format should be updated to keep it fresh."

Meanwhile, the Daily Star is reporting that it was fixed for Cher Lloyd to be sent through to the final because Simon Cowell confirmed plans for a Cheryl Cole and Lloyd duet on Saturday's Xtra Factor.