The prime minister is in danger of breaking his pledge to lead the "greenest government ever", a coalition of environmental charities and aid agencies has warned.

In an open letter to David Cameron on the first anniversary of the speech in which he made the promise, the chief executives of 15 campaign groups including Greenpeace, Friends of the Earth, Christian Aid, Oxfam and the Royal Society for the Protection of Birds said he must set out the case that a green economy was central to UK prosperity and not "a cost to be endlessly debated and watered down".

As a first step the prime minister must send a signal to investors and the public by accepting key recommendations by the government's climate advisors to sign up to a 50% cut in emissions on 1990 levels by 2025 and 60% cuts by 2030, they urged.

The call comes after reports of a split within the cabinet over whether to accept the Committee on Climate Change's recommendations on the fourth five-year "carbon budget", which will govern emissions reductions to 2027, with some departments voicing concerns that meeting the goal could hamper economic growth.

The government is likely to announce next week whether it accepts the recommendations.

The 15 organisations state that the government "started with a strong sense of purpose on the environment but is now in danger of losing its way".

As examples of a "promising start" the groups cite the cancellation of the third runway at Heathrow, the decision to set up a green investment bank and a commitment to a natural environment white paper.

But they say that elsewhere the government's green record in the past 12 months has been "less impressive". The campaigners identify a range of "real setbacks", pointing to the proposed changes in the planning system, which they claim will not provide sufficient protection for wildlife and the countryside, the weakening of the zero carbon homes policy and delays in giving borrowing powers to the new green investment bank.

They criticise the inclusion of all environmental protection law in the government's review of red tape.

The campaigners write there is still scope for Cameron to fulfil his pledge but it would require urgency, resolve and strong leadership.

"We urge you to set out the case that a green economy is central to the future prosperity of the UK," they write.

The government must prioritise strong environmental protection in planning rules, commit resources to restoring the countryside and seas and put international funding to tackle climate change and secure natural resources at the forefront of foreign policy, they add.

The letter is signed by the chief executives of Green Alliance, Christian Aid, Greenpeace, RSPB, WWF, the Campaign to Protect Rural England, the Campaign for Better Transport, Oxfam, Friends of the Earth, the Institute for European Environmental Policy, the Wildlife Trusts, Butterfly Conservation, Plantlife, the Wildfowl and Wetlands Trust and the Woodland Trust.

It follows a statement by the environment secretary, Caroline Spelman, that set out the government's "green successes since taking office" but did not reflect on policies that have been criticised by the green lobby such as the proposed forest sell-off and eventual U-turn, the huge cuts to flood and coastal defence spending, and the proposed badger cull.