From 27 May, new and existing customers on the Pioneer Variable 5.1 tariff will pay between 2.3 and 2.4 per cent more for their electricity.

The hikes will push prices to £1,239 a year for cash customers and £1,176 for those paying by direct debit.

The increases come a week after independent energy firms Ovo and First Utility fuelled whispers of a price war by introducing tariffs under £1,000 within days of one another.

Tom Lyon, energy expert at uSwitch.com, said: “Just when we thought that small suppliers were coming to the rescue, Co-op has dealt this blow to consumers. This comes completely out of the blue at a time when some suppliers are doing all they can to make energy more affordable.

“After a winter of rationing heating and hot water to cope with rising bills, this could be a bitter pill to swallow for long-suffering consumers.”

Even SSE – one of the ‘Big Six’ energy providers now facing investigation by the Competition and Markets Authority for “possible tacit co-ordination” on the size and timing of energy price rises – has pledged to freeze electricity and gas prices until 2016.

In a statement, Co-operative Energy indicated that it was passing increased costs onto consumers which it formerly had absorbed. 

Ramsay Dunning, group general manager, said: “We are not increasing prices to reward financial investors. We are simply no longer able to absorb government-imposed costs.

“We pledged to offer fair pricing and even after this increase we will still be competitively priced against the Big Six’s standard tariffs.”

Yesterday energy regulator Ofgem launched a new website – www.goenergyshopping.co.uk – to help consumers better understand their energy options.