JT has since hit back at the ruling, calling it 'surprising and disappointing', and stood firm that it did have the fastest network in the Channel Islands.
Airtel-Vodafone and Sure, which initially challenged the claim, accused their rival of 'going to great lengths to extensively mislead mobile customers' and questioned the methodology used.
Earlier this year, more than 446 speed tests were completed by people across the island as part of JT's '4G Challenge', which, it said, showed that its network was the quickest.
However, the ASA did not consider the data to be representative of the speeds customers of different networks would achieve at different times of day, at different locations, across Guernsey.
'Therefore, given how we considered consumers were likely to interpret the claim, we concluded that it had not been substantiated and was likely to mislead,' it said.
Daragh McDermott, director of corporate affairs for JT, said the company firmly believed that the result of the public independent testing was correct.
'Ultimately, it is what customers think that really matters – that's why we asked them to test all of Guernsey's 4G networks in any location of their choosing.
'The tests were conducted fairly, both by our own customers and those on other networks, and they have produced a robust result. The simple fact that more survey responses came from people living in more densely populated areas doesn't change that one bit.'
