SCOTTISH Power has been warned against price hikes after announcing it will become the first major UK energy supplier to generate all its electricity from wind power.

The Spanish-owned giant sold its last gas and hydro stations to the power company Drax for £702 million, in a move it argued would help tackle climate change and provide the best value for customers.

But critics raised concerns over a boom in “unpopular and unsightly” wind farms after the energy firm insisted it would invest £5.2 billion over the next four years to more than double its existing renewables capacity.

Scottish Conservative energy spokesman Alexander Burnett said it would “obviously be an expensive switch for Scottish Power”.

He added: “It’s important they now reassure their many customers that this will not result in increases to energy bills.

“Fuel poverty is already far too widespread in Scotland – something all energy companies need to think about – and we can’t afford for that to get any worse.”

He said the move "may well be an honourable enough objective from Scottish Power", but added: “Well-sited windfarms which have the support of those living nearby most certainly have a place in Scotland’s energy generation, and will continue to do so.

“But communities all over Scotland have endured years of the SNP forcing through unpopular and unsightly windfarm developments.

“That has an impact on house-prices, visual amenity, and a whole range of other issues.

“People, especially in countryside locations, will be very anxious about this, and are simply unwilling to have more turbines foisted upon them.”

Scottish Power hiked its standard gas and electricity prices for the second time in a year earlier this month, increasing costs by an average of 3.7 per cent.

But a spokesman insisted renewable energy sources were the “best way to generate electricity”, adding: “Onshore wind is now the cheapest form of large scale electricity generation.

“A recent report – from renewable energy consultants BVG Associates – actually demonstrated that awarding contracts for new onshore wind power could deliver a net payback to UK consumers of up to £1.6 billion.”

The company also pointed to plans to introduce a temporary price cap by the end of this year. Regulator Ofgem has said the move – which will affect consumers on poor value default deals – will see 11 million households save around £75 a year on average.

Scottish Power's latest shift will not mean its customers only receive energy generated by wind farms, as it will still buy and sell on the UK grid.

However Jamie Stewart, Citizens Advice Scotland’s energy spokesman, called on the energy giant to ensure the move will lower costs.

He said: “Our consumer surveys show that the majority of people in Scotland think that climate change needs to be addressed now. To that end we hope that Scottish Power’s focus on renewable generation can help to reduce carbon emissions.

“However energy prices have been rising steadily recently and are unaffordable for some. Scottish Power should ensure that their shift to renewable energy will reduce costs to its customers.”

Age Scotland has also raised concerns over rising energy costs, which leave older people struggling to pay their bills.

The latest announcement comes just days after the International Panel on Climate Change (IPCC) insisted the world was "off track" in efforts to limit environmentally catastrophic global warming.

It warned "rapid, far-reaching and unprecedented changes in all aspects of society" were required.

Scottish Power has sold all of its coal plants over the last decade. It has 2,700 megawatts (MW) of wind power capacity operating or under construction in the UK, and a pipeline of future projects capable of generating more than 3,000 MW.

It previously warned a surge in onshore wind farms would be necessary to cope with consumer demand.

Graham Lang, from anti-windfarm group Scotland Against Spin, said campaigners were “always concerned” about a loosening of the rules to allow more wind farms to be pushed through.

Keith Anderson, chief Executive of Scottish Power, said the company had made a “pivotal shift”.

He added: “We are leaving carbon generation behind for a renewable future powered by cheaper green energy. We have closed coal, sold gas and built enough wind to power 1.2 million homes.

"Every working day we are investing over £4m to deliver cleaner, smarter power for customers. From today we can focus solely on making energy generation cheaper, cutting carbon quicker, building smart grids and connecting customers to renewable electric future for transportation and heating.”

Ignacio Galán, chairman and chief executive of Iberdrola, Scottish Power's parent company, said: "Energy companies must be part of the solution to climate change.

"Iberdrola is acting now to cut carbon emissions 30 per cent by 2020 and be carbon neutral by 2050. The sale of these generation assets is consistent with our strategy."