Ofcom has ruled that ITV broke the broadcasting code by “materially misleading” viewers over the use of a stunt-double dog that helped Jules O’Dwyer and Matisse win the final of Britain’s Got Talent.

The broadcasting regulator received more than 1,175 complaints and launched an investigation over the use of a second dog, called Chase, to perform the high rope walk that clinched O’Dwyer the £250,000 first prize and a place in the Royal Variety Performance.

O’Dwyer narrowly won the public vote, in which 4.5m votes were cast via the BGT app for free, and by landline or mobile at 50p.

The stunt double of Britain’s Got Talent winner Matisse performs the high-rope walk during the show’s final Guardian

“After a detailed investigation, we found ITV broke broadcasting rules by misleading viewers during the series finale,” said an Ofcom spokesman. “We accepted ITV had no intention to deceive the audience. However, the presentation of the act did not make clear to viewers that a central part was performed by a second dog”.

Ofcom said that ITV broke rule 2.14 of the broadcasting code which states that “broadcasters must ensure that viewers and listeners are not materially misled about any broadcast competition or voting”.

However the media regulator did not impose any wider sanction or order ITV to refund voters who felt they had been misled.

ITV has moved to offer viewers who voted for O’Dwyer to win a refund, but did not go as far as looking to hand the title to runner up Jamie Raven.

O’Dwyer and Matisse received 22.6% of the votes cast, narrowly neating magician Raven, who drew 20.4%.

“There was never any intention to mislead viewers,” said a spokeswoman for ITV. “The majority of votes cast for Jules’ act were received through the free voting app. However, we accept that some viewers who voted for the winning act by a paid voting route may wish to seek a refund, or that the cost of their vote be donated in full to the Royal Variety charity”.

Almost 90% of the votes for O’Dwyer to win were via the free app.

The presentation of the act did not make clear to viewers that a central part was performed by a second dog Ofcom

“Broadcasters inviting viewers to vote using premium rate numbers have a particular responsibility to ensure the audience is clear about what they are voting for,” said Ofcom. “By failing to do so, we’ve concluded the presentation of the act was likely to have materially misled some people.”

ITV said that the “press furore” following the revelation of the use of a stunt dog had “unfairly detracted from her evident skills as a dog trainer and a storytelling performer, and the talents of her troupe of dogs” and that she still should be the winner.

In a response given to Ofcom during its investigation, ITV admitted that naming the act Jules O’Dwyer & Matisse, despite the use of the two other dogs Chase and Skippy, “might have contributed to some of the complaints of some viewers that they had been misled”.

ITV said it never intended to deliberately mislead viewers but a “hectic” week of live BGT shows in the run up to the final had led to the incident.

“[A] failure to anticipate that some viewers might feel misled by the staging and construction of this performance … should be viewed in the context of a hectic week of activity during the preparation and staging of six evening of live programming featuring 45 different acts.”