The warning came after Kew announced a £5m deficit for the year and plans to axe about 125 science and public engagement staff.
The job cuts come after a steep reduction in Kew's public funding from the Department of the Environment, Food and Rural Affairs first announced in 2010.
Kew’s most important global role is its plant science. It maintains the world’s premier plant and fungal collections including 30,000 living plants, one billion seeds and the DNA of 20 per cent of the world’s plant species. Without these collections and expertise, plant conservation stops.
Kew’s funding was reduced by £1.5m in 2010-11 and it expected to lose £0.5m year on year thereafter.
However, Kew has now been told to expect further cuts of up to £1.8m before the end of 2016. These cuts plus increasing costs and unpredictable self-generated revenue led to the deficit.
Kew's initial response was to look for additional funding from philanthropic and other sources to help make good the shortfall. It also stepped up its consultancy services and other commercial activities.
Prospect negotiator Julie Flanagan said: “Kew's unions expressed their scepticism that these efforts would be sufficient to make good the funding losses.
“Demands on specialist staff time for consultancy work inevitably reduce the capacity to maintain Kew's internationally-prized scientific collections or conduct the basic research that underpins much of the world's plant conservation work.
“The government’s short-sighted approach to the institution's funding is hypocritical given that its Heritage Act requires Kew to carry out its plant and fungal conservation work. Kew does this work cheaply and does it cheaper and cheaper every year.
“Successive independent reviews of the institution have praised the quality and value of its scientific work and recommended that its public funding be maintained or increased.”
Flanagan also warned of the impact on human health.
“The Earth is losing one major drug plant every two years – yet we’ve researched less than three per cent of tropical plants for new drugs.
“Eighty per cent of the world relies on plants for medicine, yet 15,000 medicinal plants are threatened with extinction worldwide. And 70 per cent of the top-selling pharmaceutical drugs are directly or indirectly derived from natural sources – primarily plants and fungi.
“Life depends on plants. The prime minister’s claim to be the ‘Greenest government ever’ is proving to be nothing but hot air,” she added.