Five leading figures in the green movement have warned David Cameron that he risks undermining both the energy security of the UK and its ability to cut greenhouse gas emissions if he follows the advice of former Friends of the Earth directors and phases out nuclear power in the UK.

Guardian columnist George Monbiot and author Mark Lynas are among the signatories of an open letter that takes issue with a letter sent to Cameron by Jonathon Porritt, Charles Secrett, Tom Burke and Tony Juniper earlier this week.

In that letter, the prime minister was told the UK is effectively subsidising French companies EDF and Areva, which are contracted to build several of the eight planned new reactors.

In response, Monbiot and his co-authors accuse the former Friends of the Earth directors of adopting a “jingoistic tone” and adhering to a “reflexive ideological opposition” to nuclear power.

Addressing David Cameron, they write: “It is abundantly obvious that the authors of the 12 March letter to you are not against nuclear power because they think it is too expensive, but that they think it is too expensive because they are already against nuclear power.”

They go on to point out the Climate Change Committee found nuclear is potentially the cheapest of all low-carbon options available until 2030, and insist nuclear and renewable energy combined is the only viable option for phasing out carbon-intensive coal-power stations.

They add that Germany and Japan, both countries that are committed to phasing out nuclear, have subsequently been forced to increase their use of fossil fuels to cover the resulting energy shortfall.

The authors also argue that pursuing such a policy in the UK could risk legal action under the Climate Change Act if emissions rise as a result.

“Nuclear remains the only viable large-scale source of low-carbon baseload power available to energy consumers in the UK today,” the letter states.

“Whilst we enthusiastically support research into new technologies, the deployment of renewables, demand-management and efficiency, these combined cannot, without the help of atomic energy, power a modern energy-hungry economy at the same time as reducing carbon emissions.”

The letter comes on the same day as a group of more than 100 business leaders and environmental campaigners wrote another open letter to the prime minister in which they called for more clarity on future support for solar energy, wind energy and biogas.

Signatories, including Richard Branson, Ecotricity founder Dale Vince, Green MP Caroline Lucas, financier Ben Goldsmith and about 65 leading figures from business, politics and NGOs, express their concern the UK is missing an opportunity to spread the ownership of the “next generation of energy infrastructure”.

The authors call for the ongoing review of renewables subsidies to recognise the importance of “simple and accessible feed-in tariff policies” and ensure that the new National Planning Policy Framework promotes a move to local and community-owned energy projects.

“In wind alone, the UK has more than 40 per cent of Europe’s renewable energy resources – enough to power up our economy three to four times over, generate exports, and provide the tools for communities and entrepreneurs to do their job,” they write.

“Bringing energy supply and demand together, a decentralised energy market can make real efficiencies in costs associated with our antiquated infrastructure and transmission loss, deliver savings for tax payers and provide frustrated investors with new opportunities.”