Invasive non-native species such as grey squirrels and Japanese knotweed cost the British economy £1.7 billion every year, a new report suggests.
The researchers found that when such species take hold, crops, ecosystems and livelihoods can suffer.
The cost to the Scottish economy is £251 million a year and to the Welsh economy £133 million, according to the study, The Economic Cost of Invasive Non-Native Species to the British Economy.
Costs to the English economy are significantly higher at £1.3 billion because more invasive non-native species (inns) have become established there.
The rabbit is the most economically damaging species, followed by Japanese knotweed.
Other creatures in the top 20 include the rat, the house mouse and the mink, along with plants such as the rhododendron and giant hogweed.
The research was carried out by international scientific organisation CABI for the Scottish Government, Defra and the Welsh Assembly Government.
Ministers said the report showed early action was the best way to tackle invasive non-native species.
Richard Benyon, Defra Minister for the Natural Environment, said: "Invasive non-native species have a significant impact on the British economy and damage our own wildlife. The costs of controlling these species will rise unless society takes steps to prevent them taking hold and spreading.
"It becomes increasingly difficult and costly to control invasive non-native species as they become more established. Taking early action may seem expensive, but this report shows that it is the most effective approach, saving money in the long run and helping our native wildlife to thrive."
