The Advertising Standards Authority (ASA) found that Collins had tweeted on behalf of hair salon chain Toni & Guy in April, but not flagged up to her 560,000 followers that they were marketing communications.
The ruling comes as brands are increasingly coming under the attention of regulators over their efforts to get high-profile people to post promotional messages on their popular social media profiles.
Sportswear giant Nike has lodged an appeal to overturn a recent ruling by the ASA that the tweets posted by two footballers should have been marked as ads.
In her two tweets, Collins had reported how great her hair looked after visiting a Toni & Guy studio in the Lakeside shopping centre.
She also encouraged her followers to get a 10% discount by referencing her name at the Lakeside salon.
The first tweet stated: "In @Toniandguylside having such a wonderful time defo got my hair back to good condition 10% off call today and quote #gemma x"
Followed by: "10% off @Toniandguylside I have the most amazeballs hair colour and condition best salon ever call and say #gemma for discount xx."
After a complainant contacted the ASA, Toni & Guy Lakeside said that Collins had visited their salon, but they had decided to waive the cost of her hair treatment.
They attempted to argue that Collins was so pleased with their service that she decided to tweet about it. She had apparently asked about offering a discount, and it was agreed that she should mention it.
Toni & Guy claimed that the tweets had been compiled by Collins on the "spur of the moment" and were not intended to be a formal advertising campaign.
However, the ASA said that without a clear identifier, such as "#ad", the tweets were not clearly marked to the public as marketing communications by Toni & Guy.
"The tweets appeared to have been written on a spontaneous visit to the salon and users could have interpreted them as referring to a pre-existing 10% off sales promotion, which Gemma Collins had herself taken advantage of on her visit," said the ASA.
"The tweets did encourage users to quote "#gemma" but, in the context of the whole tweets, users could have overlooked the significance of that or not understood that it related directly to Miss Collins.
"In the absence of an identifier such as "#ad", we considered the tweets were not obviously identifiable as Toni and Guy marketing communications and therefore concluded they breached the Code."
The regulator said that the ads must not appear again and warned Toni & Guy that it must "ensure that its advertising was obviously identifiable as such".
Meanwhile, Nike has asked for an independent review of the ASA's ban on its Twitter campaign, which was fronted by brand ambassadors Wayne Rooney and Jack Wilshire.
Nike, which was the first company in the UK to have a Twitter campaign banned, has refused to remove the tweets despite the ASA's ruling.
The company claims that the messages were part of its wider Make It Count campaign, and denies that it "misled" consumers.
