SSE has a monopoly on Total Heating Total Control (THTC) tariffs in the Highlands and Islands, and should be brought to account, according to the Western Isles Poverty Action Group.
Chairman Angus McCormack has called on Theresa May to intervene and 'address this anomaly with immediate effect'.
"The Prime Minister has stated that her government is intent on ‘fighting injustices’. The monopoly resting with SSE over THTC in the Western Isles is an injustice that needs addressing now," said Mr McCormack.
"Not content with surcharging the people of the Western Isles by 2p per unit of electricity used, it has come to light that SSE has a monopoly on Total Heating Total Control tariffs in the Highlands and Islands. This means that people on THTC are unable to switch to cheaper tariffs costing them hundreds of pounds each year.
"The government, Ofgem, the Competition and Marketing Authority (CMA) and even SSE itself insist that the market allows people to obtain lower tariffs by switching, but not if you are on THTC. 110,000 SSE customers in the North of Scotland are consequently unable to switch to cheaper tariffs. No wonder the Fuel Poverty level in the Western Isles stands at 71%."
He added: "CMA is aware of this serious anomaly but will not take action to redress the situation at least until the end of this current year. That is not good enough. It means that SSE customers on THTC will be obliged to pay hundreds of pounds more than they should. This is profoundly unfair and demonstrates that yet again the people of the Western Isles are penalised by SSE."