Of the 337 moth species studied between 1968 and 2002, two thirds showed a decreasing population trend and several fell dramatically. One species, the brown Dusky Thorn which used to be common in summer and early autumn, declined by 98 percent.

"These findings are shocking and a big wake-up call," said Martin Warren, chief executive of Butterfly Conservation. "No one suspected so many species were declining so rapidly.

"Moths are a key part of the food chain as most birds rely on insects for part of their diet. For so many moths to disappear shows a widespread and severe crisis for Britain's natural heritage."

In a preface to the report, "The State of Britain's Larger Moths," naturalist and broadcaster David Attenborough said the results of the study were "significant and worrying" as moths were valuable indicators of what was happening in the British countryside.

"Although the precise causes of these losses still need to be uncovered, the findings set more alarm bells ringing about the extent of human impact on our environment," he said.

"Moths are important in food chains and their declines may have significant knock-on effects on many animals, such as birds bats and invertebrates."

Britain has around 2,500 species of moths, which are closely related to butterflies. Most moths are nocturnal.

Scientists produced the study by collating as many as 8 million reports from volunteers operating night moth traps at 430 sites across Britain.

Warren said the exact reasons for the decline in moth populations were unclear but some causes were easy to identify.

Half of Britain's ancient hedgerows, a key breeding ground and reserve for many species of insects and birds, had been destroyed since World War Two.

Moth population levels were also "strongly correlated" with large-scale climatic changes with numbers decreasing after wet winters and warm springs. Since the study began "climate change has become evident," the report said.

Moths were also declining particularly quickly in urban areas, which could be related to "light pollution" from all-night lighting, a recent study has suggested.

"The prime suspects are habitat loss and climate change," Warren said. "We should stop using pesticides in our gardens, leave rough areas and protect the countryside, especially wild habitats."