One person challenged whether the Twitter comments made by Collins were marketing communications, and should therefore be identified as such.
"In @Toniandguylside having such a wonderful time defo got my hair back to good condition 10% off call today and quote #gemma x," said one tweet published by Collins, while the other read, "10% off @Toniandguylside I have the most amazeballs hair colour and condition best salon ever call and say #gemma for discount xx".
Collins has more than 564,000 followers on the micro blogging site.
Toni and Guy Lakeside told the ASA that the Towie cast member had visited the salon for an appointment and that they had decided to waive the cost of their services.
As she had been pleased with the service provided, it was suggested that she tweet about it. The salon said Gemma had asked whether she should mention a discount and it was agreed that a discount should be offered.
However, Toni and Guy said the tweets had been compiled by Gemma on the spur of the moment and that they were not part of any formal advertising campaign.
While the messages did not include the terms "marketing", "sponsored" or "advertisement", the salon felt the mention of a 10 percent discount made it clear that the tweets were marketing communications.
The complaint was upheld by the ASA, which noted that the CAP Code did not just require ads to be identifiable as marketing communications but that they must be obviously identifiable as such.
The ad breached rules 2.1, 2.3 and 2.4 of the CAP Code Edition 12, which covers recognition of marketing communications.
"The ASA considered the average Twitter user would follow a number of people on the site and they would receive a number of tweets throughout the day, which they might scroll through quickly," the ruling said.
"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."
Had the tweets included the identifying hashtag #ad they would not have breached advertising rules.
The ASA ruled that the ads must no longer appear and told Toni & Guy to ensure that its advertising was obviously identifiable in the future.