Guernsey's competition regulator has accused Sure and JT of an 'illegal' deal to divide up control of the Channel Islands' mobile networks between them.
The GCRA says the agreement between the two operators went 'well beyond' legitimate reasons.
In its final decision after a year-long investigation, the GCRA says the two companies colluded, forming a plan that would see JT remove its Guernsey mobile masts and use Sure's - in exchange for Sure doing the same in Jersey.
It would have given each company greater control of their 'home' island's infrastructure.
Guernsey's competition watchdog has taken a different approach to the JCRA in Jersey, which after an investigation said it welcomed the companies' plans to reduce duplication.
The regulator also claims the two firms exchanged commercial information, even agreeing to introduce new 5G upgrades at a slower pace.
The GCRA says Airtel-Vodafone, the island's second-biggest operator, wasn't privy to the deal.
Sure has refuted any allegation of anti-competitive behaviour and says it plans on appealing through the Royal Court.
CEO Alistair Beak says the company 'absolutely disagrees' with the GCRA's final decision:
"At all times Sure has acted in accordance with the spirit and letter of the law. We are very confident that this will be confirmed in due course following the appeal which will now be filed against this decision.
"Earlier this year we submitted a thorough and detailed rebuttal to the GCRA’s Draft Decision, which we supplemented with a presentation to the GCRA in September. These representations clearly demonstrated that Sure had not contravened competition law when it engaged in the discussions with JT to explore the idea of a single 5G mobile network for Guernsey. This was done in response to what was then the latest iteration of the GCRA's allegations. We are disturbed to see that, yet again, there is now another iteration of the allegations and one which was not put to us before. It is clear that the GCRA is determined to justify its lengthy and expensive investigation by portraying Sure as having acted wrongfully in some way, shape or form and to do so regardless of the evidence provided to it.
Sure CEO, Alistair Beak
"Their approach is unfair and deeply flawed. It demonstrates a lack of ownership on the part of the GCRA for their conduct and role in these discussions, and a lack of regard for the instructions issued by the Government at the time. The fact is that discussions with JT only took place after Sure was instructed in clear and unequivocal terms by the regulator and Government to have those discussions.
"We, therefore, find it astonishing that the GCRA should seek to criticise Sure. Their approach to this matter throughout has lacked principle, fairness and reason.
"This case began more than two years ago; from the outset, we have cooperated fully, at enormous cost in money and time. Our focus is on connecting our island communities for a better future. If left unchallenged, the GCRA's approach will be deeply damaging to the Guernsey business community."
JT has also denied any wrongdoing. The company's Director of Corporate Affairs, Tom Noel, says it takes the regulator's comments seriously:
"JT has and always will be fully committed to ensuring that the market is fair and open and operates in the interests of consumers. We note the stance now being taken by the GCRA with concern and, frankly, surprise.
"We won’t be making any further comment while we consider our next steps."