A promotional campaign of billboards featuring the tagline "Bigger. Fatter. Gypsier." sparked 372 complaints back in February, while an investigation was launched in May.
The adverts were deemed to be "offensive and irresponsible".
The Advertising Standards Authority (ASA) said at the time that the adverts "might not be to everyone's taste", but they were unlikely to cause any serious offence, as they were used in the context of the popular show.
However, an independent review has asked the watchdog to reopen the investigation, after an inquest was held by the Irish Traveller Movement in Britain.
The inquest stated that the billboards were racially insensitive and led to the bullying of children in their community.
The watchdog ruled that the advertisements were irresponsible by showing a child in a sexualised way, while showing another young boy "in a manner likely to be seen as aggressive".
A Channel 4 spokesman said: "It was not Channel 4's intention for these adverts to cause offence but we are sorry this was the case among some members of the gypsy and traveller community.
"All responsible action was taken pre-publication."
Ofcom dismissed several complaints made about the show in general back in March.