A consumer watchdog claims electricity companies are charging Welsh customers more for electricity.
Consumer Focus Wales say the practice is particularly marked in Scottish Power's pricing policy.
It means former Manweb customers in north Wales pay up to £75 per year more than people living in north west England.
Scottish Power said it was committed to offering fair and competitive prices for all customers.
Speaking on BBC Radio Cymru's current affairs programme Manylu, Ceri Williams, senior policy officer for Consumer Focus, said the organisation had raised the matter with Scottish Power, and with the energy regulator Ofgem.
Scottish Power said it was committed to offering fair and competitive prices to all our customers.
"Earlier this year, we reduced gas prices by an average of 8%, the largest cut of any supplier this year," said a spokesman.
"We have an ongoing programme to review the cost of prices which vary due to different costs of supplying energy to different regions."
Experts say that people who have not changed tariffs often pay more for gas and electricity while companies offer lower tariffs to people outside their original area, in order to attract customers.
According to Consumer Focus' research customers in North and South Wales could be paying around £70 a year more than their neighbours in North West England and the Midlands.
In a statement to Manylu, Jane Davidson, environment minister, said that the Welsh Assembly Government remains committed to eradicating fuel poverty in Wales.
But that several of the factors affecting fuel poverty, energy prices and income levels, were outside the Welsh Assembly Government's control.
Ms Davidson said that she was concerned that households should not pay more than necessary for energy and that it was essential that energy suppliers passed on reductions in wholesale prices to consumers as soon as possible.
Manylu on BBC Radio Cymru just after 1800 BST on Monday 7 June.