An independent Scotland will have to decide whether to classify thousands of species that only occur south of the border as alien, according to a leading wildlife expert.

Professor Chris Thomas, a biologist from the University of York, is warning that labelling as non-native birds, butterflies and other species that move north because of global warming would create "one big headache".

He points out that other countries signed up to international agreements to protect wildlife have developed policies for non-native species.

But Mr Thomas argues that it would be a bad idea to define English plants and animals as alien, and resist their arrival in Scotland.

The rising temperatures caused by climate pollution are driving many species north, he says, and they should be welcomed.

Among the species that could cross the border is the Essex skipper butterfly, which is seen as an invasive species in North America. Historically a native of southern England, it has been spreading northwards.

Birds that may be northward bound from England include the Dartford warbler, Cetti's warbler and the avocet.

In June it was reported that the tree bumblebee had arrived in eastern ­Scotland for the first time from England.

"It would be very important that an independent Scotland should welcome biological invaders from England, rather than ­classify thousands of ­British-but-not-Scottish species as foreign and be either ambivalent towards them or - worse - antagonistic," he said.