It also failed to demonstrate that those prices are ‘transparent, non-discriminatory or cost-justified’, the Guernsey Competition & Regulatory Authority said.
The electricity company had argued that it was not required to publish what it termed special agreements, but this was rejected.
It now has three weeks to publicise the prices it has been charging.
Michael Byrne, GCRA CEO said: ‘Rules of transparency are in place to ensure fair play in a special context where a single business has a powerful position in our economy with legacy advantages over any nascent competition and a very powerful position over consumers.
‘Transparency about Guernsey Electricity’s prices is vitally important to assure fairness. We have no view whether these special prices Guernsey Electricity charges a select group of customers are unfair or whether householders are subsidising those prices, and we are not saying GEL cannot compete. Guernsey Electricity’s decision not to be transparent has however removed visibility of its pricing behaviour and this cannot be allowed to continue.’
Despite being informed that its conduct breaches its electricity licence, GEL has not accepted the GCRA provisional decision.
Mr Byrne added: ‘The GCRA has indicated to GEL that this is not an acceptable situation but regrettably is placed in a position of having to enforce transparency.’