A telecoms firm "disadvantaged" Jersey customers with prices which "will choke off rivals", regulators have found.
An investigation into JT was launched in 2014 after the company raised broadband wholesale prices and removed cheaper options for customers.
The firm has been given two months to submit new prices to the Channel Island Competition and Regulatory Authorities (CICRA).
JT said CICRA had come to the "wrong conclusion".
"JT's prices at both a wholesale [to other operators] and a retail [to consumers] level are either the same, or lower, than those charged by the network owner in Guernsey", corporate affairs director Daragh McDermott said.
"We won't be deflected by these continual attacks from the regulator and will take all measures we need to defend the quality of our services."
The investigation was prompted by claims from JT's rivals, Sure and Newtel, that hikes in monthly wholesale line rental rates last year meant it was unviable for them to provide similar services on the JT-owned network.
"Margin squeeze, if allowed to continue, will choke off JT's rivals and return the market to a monopoly, reversing progress made to introduce competition over the last decade", CICRA chief executive Michael Byrne said.
"If allowed to continue without intervention by the regulator consumer choice will be damaged in a fundamental part of our everyday lives and critical to future economic growth", he said.
The States-owned company must now rectify the damage it has done to competition, CICRA said.