An all-party parliamentary group (APPG) has called for the Office of Fair Trading (OFT) to repeat its inquiry into allegations of price fixing in the "off-grid" energy market.
An estimated four million UK households – 15% of the total population – are not on the mains supply, many in remote communities away from towns and cities.
Devon and Cornwall are thought to have a higher proportion of households relying on domestic gas tanks and heating oil.
Customers have complained of exorbitant prices and unfair practices by firms that have an effective monopoly in some of the country's more remote regions.
The OFT took action against some companies in 2011, but it ultimately concluded that there was no need to regulate prices as competition was working.
But Cornwall MP Sarah Newton, a member of the APPG, said the inquiry "did not look deeply enough into what is a widespread issue".
The Conservative MP for Truro and Falmouth added: "As our new report shows, now is the time for the Government to look again at off-grid households, and take action to end the unfairness whereby such households pay up to 120% more than households connected to mains gas. I will continue to work closely with my colleagues to tackle this disparity and to secure a fairer deal for the many off-grid households in my constituency, and across the UK."
The APPG's inquiry raised concerns that the OFT's methodology has given an inaccurate picture of regional supplier dominance.
Among the report's recommendations was a call for off-gas grid pensioners to be paid their winter fuel payment early to allow them to buy heating oil in the summer and autumn, when prices are lower.
The APPG's co-chairman Therese Coffey said: "If you live off the gas grid, the chances are your energy efficiency will be worse, your fuel bills may be over twice as high, and you are more likely to be in fuel poverty.
Yet you will not enjoy the consumer protection that mains gas users do and vulnerable households suffer. We think that needs to change."
An OFT spokesman said: "We stand by the findings of our report. We have taken action to ensure companies are more transparent in their pricing practices, do not change their prices between order and delivery and to change their contract terms."
