George Eustice made comments to Western Morning News just before being appointed to new ministerial role

A new environment minister who advised David Cameron on energy has suggested subsidies for wind and solar power are "still too high", amid growing speculation the Tories are planning an assault against green costs on energy bills.

George Eustice, who had an energy brief on Cameron's policy board until this week, argued the current subsidy system needs to be replaced because it is not getting the best deal for consumers.

He made the comments to the Western Morning News just before being appointed to a new ministerial role in charge of the natural environment, water and rural affairs.

"There is an argument that the level of subsidy paid for wind and solar is still too high and this is being passed on to electricity bills," he told the newspaper.

"We need to replace the current subsidy system with a highly competitive 'Dutch auction' or tender process where renewable energy developers fight one another for the right to install new capacity at the cheapest possible price."

On Wednesday Eustice said his remarks referred to the government's desire to move eventually to an auction system for renewable subsidies and stressed they were made before he was appointed as a minister.

His comments come amid speculation the Conservatives will try to reduce green costs on electricity bills to counter Labour's promise of an energy price freeze. The move would be strongly resisted by the Liberal Democrats and environmental campaigners if it in any way involves backing away from the UK's commitment to reducing carbon emissions to tackle climate change.

According to senior sources, the Lib Dems fought off an attempt by the Treasury last year to water down the energy companies obligation - a scheme aimed at reducing energy usage and supporting people living in fuel poverty that adds to electricity bills.

On Wednesday David Cameron fuelled suspicions the Conservatives could launch a new drive against environmental costs as he criticised Ed Miliband, the Labour leader, for bringing in too many green regulations and supporting a "decarbonisation" target when energy secretary. He claimed these have pushed up bills, although the coalition's own energy advisers, the committee on climate change, argue switching to low-carbon technologies will save consumers between £25bn and £100bn.

Miliband mocked the prime minister for not knowing how to respond to Labour's promise to freeze energy bills, suggesting Cameron is unable to decide whether it is a "good idea" or "communist plot".

He also hit back with claims that only one in 10 people will benefit from the government promise to make sure people are put on the lowest energy tariffs, as it will only apply to those stuck on expensive "closed" deals. He accused the prime minister of having no idea about how to bring down fuel prices.

A spokesman for the Department of Energy and Climate Change said Miliband's claim was misleading as more than one in 10 people would be on closed tariffs after the government's reforms to the market.