The letter says the coalition should promote a green economy with "urgency and resolve" if it is to follow its vow to be the "greenest government ever".
The groups include Greenpeace, Friends of the Earth and the RSPB.
Downing Street says it stands by its record of protecting the environment and delivering a low carbon economy.
A year ago, David Cameron said the environment would be a top priority.
In their letter the campaign groups describe the pledge as a "great ambition" and 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 as examples of a "promising start".
They say the coalition "started with a strong sense of purpose on the environment but is now in danger of losing its way".
They point to the proposed changes in the planning system which they claim will not provide enough protection for wildlife and the countryside.
The letter also suggests a zero carbon homes policy has been weakened and point to delays in giving borrowing powers to the new green investment bank.
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