Using government figures, the party claimed that four out of ten households have not switched from their incumbent supplier after privatisation.
Labour said that on average, consumers that have not switched pay £28 a year more for electricity and £75.50 more a year on gas.
Shadow energy secretary Caroline Flint said: “It’s outrageous that energy companies are cashing in on their most loyal customers.
“In other industries companies have all kinds of schemes to reward their most loyal customers, but in the energy market loyal customers pay most.
“It is difficult to explain the reasoning behind these price variations other than the fact that suppliers are charging what they think they can get away with.”
The major energy suppliers denied that they are overcharging their customers they inherited from their incumbent regions.
EDF Energy said the regional variations in electricity prices is down to differing distribution charges, with the difference between the most expensive and cheapest regions being 4 per cent.
In a statement, EDF Energy added: “We have consistently offered our customer some of most competitive prices within the industry for both our standard variable and fixed priced tariffs.”
A spokesperson from Npower said: “We urge all customers to check online to ensure they are on the best tariff for their needs.
“We also have a section on our bills which shows our customers the tariff they’re on and provides a number for them to ring and check that this is the best tariff for them.”
A spokesperson from the Department of Energy and Climate Change said: “Government is acting now to help people with their energy bills, introducing more competition and transparency in the market to keep prices down and force energy companies to be honest about the profits they are making.
“We’re also working with energy suppliers so that people will be able to switch providers in 24 hours.
“Ofgem has already set out what a functioning competitive energy market should look like, and will be presenting its findings on the UK energy market in March.
“In addition, Ofgem now have the power to push for criminal sanctions if they find evidence that the energy market has been manipulated.”