The two plants in the south of England would run on palm oil, which in many cases is sourced from plantations sited on cleared rainforest land.
They are among more than 30 new power stations at various stages of the planning process that propose to burn wood and other plant materials, as owners try to take advantage of hundreds of millions of pounds of subsidies to cut reduce greenhouse gas emissions.
However, experts have warned that as well as being linked to tropical rainforest destruction – and the survival threat to the orang-utan in the wild – crop-fuelled power stations will push up food prices by competing for land, and in many cases will not even cut greenhouse gas emissions.
UK government policy insists companies use only "sustainable" sources, but campaigners claim the rules are too relaxed and it is too hard to enforce such standards on plantations far from the UK, especially when other – non-certified – land could be cleared of forests to make way for displaced food growing.
"The trouble is their sustainability standards amount to little more than greenwash: they claim to set a standard but all they are saying is you need to comply with a couple of criteria in European law that are vague," said Harry Huyton, head of climate change for the RSPB. "Once you build them [the plants] they are here to stay: they have a shelf-life of 30 years or longer and they need biofuel to feed them."
Campaigners are now urging ministers to review the policy on bioenergy that is currently expected to deliver more than half of the government's target to generate 15% of energy from renewable sources by 2020.
Doubts about the policy are made more likely by the appointment of David MacKay as chief scientist at the Department of Energy and Climate Change (Decc). Before he took the job in 2009, MacKay wrote a book on sustainable energy which said biofuels often competed with other land uses and had questionable environmental benefits, adding: "Biofuels made from plants, in a European country like Britain, can deliver so little power, I think they are scarcely worth talking about."
The energy company W4B has been given permission to build a 17.8MW plant at Portland in Dorset to burn palm sterin – a by-product of palm oil – and oil from jatropha plants grown in tropical areas of Africa. It is also waiting for the result of an appeal over a second such 50MW plant near Bristol.
The Salisbury-registered company website says it hopes construction will begin on one or both plants next year.
The two W4B plants are among more than 30 bioenergy plants which the campaign group Biofuel Watch has recorded as between proposal and construction stages – the vast majority of which would burn solid fuel – and are being built near ports including Liverpool, Swansea, Southampton, Tilbury and Teeside. According to the group, another seven are in operation already, and one has been rejected.
A forthcoming report by the RSPB estimates that by 2020 UK bioenergy plants would need imports of 19m tonnes of fuel and be paid subsidies of £1.2bn a year, equivalent to around £40 per household per year.
The chief criticisms of the plants are that there is too little space to grow the crops without creating huge pressure to clear forests and other land which could be used to grow food; and that in many cases they will generate more carbon dioxide than they will save. That is because it will take many years – sometimes centuries – of regrowth to "offset" emissions from burning wood and other plants in the generators. Where there is limited space for fast-growing woody crops on otherwise unusable agricultural land, they should be used for biofuel for transport, especial aeroplanes, for which there are fewer alternatives, say critics.
The opposition is backed by a number of recent expert statements questioning the assumption by many governments and international treaties – including the European Union and the United Nations' Kyoto protocol – that biomass is "carbon neutral", also because of the varying speed of regrowth. These include a letter signed by 90 leading scientists in the US, and a report by Austrian-based Joanneum Research, who also work with IEA Bioenergy, a research and development organisation set up by the International Energy Agency.
The Guardian could not contact W4B, but minutes of the Weymouth and Portland council planning meeting in January, after which the Portland plant got the go-ahead, show the company described some criticisms as "extremist views". Fuel oil would not be taken from food products, development would benefit third world countries, and a local monitoring group could make sure people were happy about the fuel sources, it said. Longer term, the company hopes to use algae for fuel.
A spokesperson for Decc said: "It is essential that all feedstocks for energy generation are responsibly and sustainably produced. We are introducing mandatory standards on sustainability that come into force next year. These standards apply to all feedstocks, wherever they were grown."
In a more unusual protest, 600 workers at a wood panel plant in north Wales last week took part in a Europe-wide protest against biomass subsidies, which they claim threaten a worldwide shortage of timber and the future of millions of European jobs in the industry.