Fuel poverty activists slated Scottish Power's bosses yesterday as they became the latest board to blanche over Labour's proposed tariff cap.

The firm's chief corporate officer Keith Anderson invoked shareholders' "doubts and fears" in an open letter to Labour leader Ed Miliband urging him to abandon the pledge.

The would-be PM has promised a price freeze on all gas and electricity bills until 2017 if Labour wins the next election - a move that would save the average household around £120 a year on current projections.

Consumer groups and charities have welcomed the move, with around one in five households living in fuel poverty and the average household bill now topping £1,353 a year.

But Mr Anderson yesterday insisted that his company, which doubled its pre-tax profits in 2012 to £712 million following a 7 per cent tariff hike, would have to reconsider its planned renewable energy projects.

"To the extent such a freeze would cause investors to doubt they will receive an adequate return or to fear future similar interventions, those doubts and fears would be reflected in the appetite to invest.

"As an international energy company, we carefully analyse all of the major markets in the world.

"Maintaining principles of sound regulation and avoiding regulatory uncertainty are critical to securing this global investment in the UK," he said.

Meanwhile its rival SSE's chief executive Alistair Phillips-Davies told Westminster's party leaders that they could not afford to break the laissez-faire political consensus.

"Energy policy is too important to become paralysed by politics," he said.

But Fuel Poverty Action's Maddy Jones dismissed the comments as "the latest in a long line of threats."

Energy policy was too important to be left to profiteers, she countered.

"Even if they do go on to invest in the energy system, it will be an investment focused on maintaining their profits - not creating an energy system that works for people who are currently choosing between heating and eating.

"We think the real either/or scenario here is this - either continue to see bills spiral, costing people their lives, or take control of our energy system away from the Big Six, moving to a democratically controlled system of renewables which will be cheaper and more sustainable in the long term," she said.