Butterflies are at greater risk of extinction than lions and tigers, warns study.
The biggest study of invertebrates ever conducted found that one in five is at risk of dying out. This can affect humans by threatening crops and food supplies.
Prof Jonathan Baillie, the director of conservation at the Zoological Society of London, said insects, slugs and snails may not be as glamorous as lions or dolphins but are just as important to providing the food we eat and the countryside we love.
"If they disappear, humans could soon follow," he said.
In Britain, critically endangered invertebrates include species of bumblebee that keep the countryside full of flowers and freshwater pearl mussels that filter the water in our rivers.
The society blamed pollution, loss of habitat and climate change for killing off invertebrates and urged the public to take action by demanding that food is farmed in a more sustainable way and growing insect-friendly plants in their garden.
Previous international studies have focused on groups of animals such as mammals and birds. As invertebrates make up 97 percent of species, the International Union for the Conservation of Nature, the UN body in charge of ensuring that animals do not become extinct, asked the society to lead a new study.
More than 12,000 species, from giant squid to midges, were studied around the world.
There are 1.5 million known invertebrates, although eight million species are thought to exist even if not yet discovered. Prof Baillie said scientists were shocked by the results.
"We knew that roughly one fifth of vertebrates and plants were threatened with extinction, but it was not clear if this was representative of the small spineless creatures that make up the majority of life on the planet," he said.
"The initial findings in this report indicate that 20 per cent of all species may be threatened."
The most threatened group are freshwater invertebrates because of the invasive signal crayfish from America.
Dams and pollution from nitrates used in fertilisers on fields have also played a part.
Marine species, including foods such as shrimp, squid and crab, are most at risk of ocean acidification, which threatens shell-building molluscs and coral.
Insects on land are being driven out by human development such as cities and pollution caused by cars and factories.
All species are in danger from further habitat loss caused by climate change.
Prof Baillie said that invertebrates help store carbon at the bottom of the ocean, filter water, decompose waste, pollinate important crops and are an essential part of the food "web", ultimately feeding hundreds of millions of people.