The Ministry of Defence (MOD) has been told it needs to make "major changes" if it is to hit the Government's environmental targets by 2050.
A report by the National Audit Office (NAO) said the department will need to change how its equipment and estates are run if it is to achieve net zero green gas emissions.
It warned "almost all" vehicles and weapons in use, or under procurement, rely on fossil fuels and are expected to still be in operation in 2050.
It said the MOD is responsible for 50% of the Government's emissions but found it has also made progress.
The report said the department has reduced its emissions by 42% over the last decade – hitting the target set by Government.
However, the report also found that emissions from military operations – not included in the Government target – were reducing at a "slower rate".
Between 2018 and 2019, 1.8 million tonnes of carbon emissions were produced during defence operations, burning 666 million litres of fuel.
An MOD spokesperson said: "We have a good record on environmental protection and have worked hard to introduce sustainability measures in recent years.
"We know we must go further and faster and will publish an ambitious climate change strategy later in the year that will outline how we will continue to meet defence demands while achieving Government targets."
While the report said the MOD was on course to hit targets for waste reduction and waste to landfill, it warned that the department faces challenges achieving other targets including waste recycling and paper use.
It also warned the MOD needs to improve the efficiencies of its buildings and increase its number of ultra-low-emission vehicles.
It said the department should be using 1,700 by December 2022 but is currently only leasing 12.
Last year, the Chief of the General Staff said the Army must become more eco-friendly to attract future recruits.
General Sir Mark Carleton-Smith said the Army's current equipment programme is "possibly the last to be dependent on fossil fuel engines".
The MOD said it will examine some of the report's findings in its upcoming Security, Defence and Foreign Policy Review.