Beavers living in the wild across Tayside for a decade, were once native to Britain, are free from disease but still pose threats to agriculture, new reports confirm.

The species was hunted to extinction in Scotland about 400 years ago, but now more than 150 are prospering in the region, having escaped or been illegally released from private collections. This is thought to have been around 2006.

Since they have been found in rivers and lochs stretching from Kinloch Rannoch, Kenmore and Crieff in the west to Forfar, Perth and Bridge of Earn in the east.

In 2012 rather than order the culling of the population, then estimated to be 100, Scotland's environment minister Stewart Stevenson, opted to allow the Tayside beavers to remain in the wild. This for the duration of the official trial reintroduction of beavers in Knapdale in Argyll.

The Scottish Gamekeepers Association (SGA) warned the decision could lead to major flooding, causing agricultural damage and forestry problems. The SGA called for more research.

At Mr Stevenson's request, the Tayside Beaver Study Group (including farmers, scientists, forestry, angling and environmental bodies) was set up to monitor their impact on land use. Its findings, along with those from the Knapdale trial and other research, will help Scottish Ministers decide later this year whether or not to permanently reintroduce beavers to Scotland.

Three reports are now published by Scottish Natural Heritage (SNH) on behalf of the study group. One includes genetic tests on 25 of the animals which show the beavers to be Eurasian therefore once native, rather than the alien North American species.

A second shows they are all healthy and free from any parasites or diseases of concern to humans, domestic animals and other wildlife.

The final report shows the beavers are successfully producing young and still spreading through the Tayside catchment.

Their most significant impact has been on farming, especially in the intensively cultivated arable ground on the flood plain of the lower River Isla where it meets the River Tay.

Beaver dams left in place at this location could "cause the extensive network of drainage ditches to fail, causing flooding and interfering with cultivation of productive land. Beaver burrows in earth flood banks also increased the risk of a breach and flooding of the farm land behind," the report says.

However a number of methods to protect trees from being gnawed and felled and to reduce water levels behind dams, have been trialled successfully. But "The impacts of burrowing in flood banks and regular damming of drainage networks were more challenging to manage. "

These issues would need to be addressed effectively if beavers were to remain, the study group says.

David Bale, Chair of the group and SNH's Area Manager for Tayside & Grampian, said: "These are very useful findings. They show there is no evident risk of diseases being transmitted from the Tayside beavers to other animals, or indeed to humans. The genetic tests tell us that they would be suitable for permanent reintroduction to Scotland, because they are Eurasian rather than North American beavers. They are also varied enough genetically to make a reasonable first step towards a full reintroduction if that was the decision of the Scottish Government. "

In many situations, beavers were likely to cause few concerns. "But if they were to be permanently reintroduced, efficient, effective and affordable ways of managing and reducing potentially significant impacts on intensively farmed land and other interests would have to be found, " he said.