Dr Richard Benwell, chief executive of Wildlife and Countryside Link, said: “Invasive species are one of the biggest threats to wildlife. They are also bad for business, affecting both the fishing, agriculture and leisure industries.

“Here in the UK, some of our most cherished species, from red squirrels to junipers, are suffering huge declines as they face disease, predation and competition from invasive species.”

He added: “Prevention is better than cure, so it’s important to stop other invasive species before they gain a foothold.”

Invasive species are costing the economy £1.86billion a year and, from next month, all trees funded under government tree planting schemes will have to meet new biosecurity requirements.

But environmentalists want the requirements extended to all trees and potted plants entering the country, while imported wood, soil and compost should be heat-treated to get rid of the pests.

They also called for the expansion of the cultivation of British trees and plants for domestic sales to reduce import dependency on horticultural products.

Things people can do to help

Dr Paul Walton, head of habitats and species at RSPB Scotland, said there were simple things people can do to help.

“Avoiding imported garden plants and compost – and using peat, of course – is one of them,” he said.

“But we also need the government to drive change in the horticulture industry to help stop the encroaching hitchhikers.”

A spokesperson for the Department of the Environment (Defra) said: “We continuously assess changing threats to plant biosecurity, with a robust regime to minimize risk, strengthen safeguards and meet Organization standards. World Trade on Protective Measures.”