Charity Viva! had featured an advert with a pig called Hope, which was shown bounding through a grassy field after being rescued by the group.
The mood of the advert then changes, showing pigs crowded in a cramped pen and trapped behind bars as a voiceover makes the claim that 90 per cent of pigs are factory farmed.
The National Pig Association and seven members of the public contacted the advertising watchdog to question the veracity of the claims, which were aired to audiences before screenings of Jumanji: Welcome To The Jungle and Star Wars: The Last Jedi.
The Advertising Standards Authority (ASA) upheld a complaint against Viva! and said concerns had been raised that the factory farming statistic was "misleading", while the advert also featured methods of farming that are outlawed in the UK.
The Department for Environment, Food and Rural Affairs (Defra) states in its code of recommendations on the welfare of livestock that "a pig shall be free to turn round without difficulty at all times".
Complainants told the watchdog that the statistic implied "the vast majority of pigs farmed in the UK were restricted to the indoors and never experienced the outside".
Viva! responded that "if anything" the figure was an underestimate, calculating that 7.98 per cent of pigs were outside for at least the first part of their lives, based on Defra figures.
The ASA rejected this analysis, pointing to data from the RSPCA and Compassion in World Farming which showed 40 per cent to 42 per cent of intensively or factory farmed breeding sows were kept outdoors, with piglets staying with them until weaning was complete at between 21 and 28 days old.
It said in its ruling: "For those pigs and piglets, while they were farmed intensively, we considered it was not true to say that they would never experience life outside.
"We concluded therefore that Viva! had not substantiated the claim and that it was likely to mislead."
Regarding the claim that the advert featured outlawed farming practices, Viva! said the cramped conditions were still used legally at many farms when sows were artificially impregnated.
The watchdog said the advert actually implied intensively or factory-farmed pigs would be kept in similar conditions "most of the time".
It accepted that pigs were sometimes kept in such surroundings in accordance with Defra guidance, but added: "The dark lighting and close-up and brief nature of some of the footage meant that it was not clear why pigs were being accommodated in the way shown or for how long."
The advert was also ruled to be misleading on the second count.
A third complaint – that the advert caused distress without justification – was not upheld.
The ruling concluded: "The ad must not appear again in the form complained of.
"We told Viva! to ensure their ads did not mislead about, for example, how animals would be accommodated in intensive farming environments."
Agencies contributed to this report
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