The broadcasting watchdog received 1,150 complaints after it was revealed trainer Jules O'Dwyer used her other dog, Chase, to perform a tightrope walk because her Border Collie, Matisse, was afraid of heights.
An Ofcom spokesman said: "We've opened an investigation to determine whether viewers of ITV's Britain's Got Talent final, who may have paid money to vote, were misled about the competition."
A total of 4.5m votes were cast on the final night, but after the backlash some days later, Jules said in an interview: "I was just putting on a show, it was not my intention to deceive people. We have never hidden the fact we used a multitude of dogs."
A BGT spokesperson said at the time: "We are sorry if this was not made clearer to the judges and viewers at home during their final performance."
Despite Jules insisting she wasn't trying to deceive viewers, show boss and head judge Simon Cowell said he "felt like a bit of an idiot". He told the Daily Mirror: "The moment I found out, I literally put my head in my hands."
However, Simon still said she was a deserving winner: "I want people to feel proud of her. She did well, she should be happy."
Meawnwhile, ITV director Peter Fincham said at the Voice of the Listener and Viewer's spring conference earlier this month: "In the semi-final it was made quite clear the other dog played a role in it. In hindsight, in the final it would have been better if that was clear."
The winner of BGT gets a £250,000 prize and a spot performing on the Royal Variety Show.
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