Farmers and landowners have been inhumanely shooting beavers causing them to suffer slow, painful deaths, and their babies to starve, according to official port-mortem reports.

Autopsies of 21 beavers shot dead around Tayside since 2010 reveal that two were pregnant and two were feeding their young. Three were shot with low-calibre guns, or from too far away to ensure instant death.

Roisin Campbell-Palmer, the conservation projects manager at the Royal Zoological Society of Scotland (RZSS), has described the figures in a private email as just “the tip of the iceberg” of a “growing” and “depressing” tendency to kill beavers. RZSS, which conducted the post-mortems, had to buy “a new freezer for beaver bodies”, she said.

The revelations have shocked campaigners and politicians, who are demanding urgent action by Scottish ministers to end the cruelty. Farming and landowning groups have defended the need to kill beavers, but say it should be done humanely.

Beavers were hunted to extinction in Scotland in the 16th century, but some escaped captivity or were illegally released into the wild on Tayside about ten years ago. There are now reckoned to be over 150 living on the Tay, the Earn, the Isla and other rivers and burns.

But because they were not legally introduced, there are no rules preventing them from being killed in the breeding season, or specifying the type of firearms that should be used. Such rules, including closed seasons for shooting, are common to prevent unnecessary cruelty when culling other wild animals.

Internal emails released under freedom of information law to BBC Scotland reveal that RZSS has been unsuccessfully urging the introduction of a closed season for beavers since last summer. In a letter on August 12 to the government’s wildlife agency, Scottish Natural Heritage (SNH), RZSS highlighted a series of “disturbing findings” from recent post-mortems.

One adult female autopsied was found to have recently given birth to four young, known as kits. “We wish to flag up the welfare concern that these kits most likely later died through starvation,” said RZSS, which is based at Edinburgh Zoo.

An autopsy report for another female beaver on July 15 2015 concluded that it had been shot from too far away to ensure an instantaneous kill. “It may have taken several minutes to die,” the report said.

Other post-mortems found dead foetuses in female beavers that had been killed. RZSS told the Sunday Herald that it had raised “welfare concerns” directly with the Scottish Government, but it was still awaiting a response.

Libby Anderson, policy advisor to the animal welfare group OneKind, pointed out that the beavers were not to blame for their unauthorised presence on Tayside. “It is shocking to hear of these harmless animals being inexpertly shot and suffering slow, painful deaths, and that young kits have been left to starve,” she said.

“Further delay in deciding their legal status only exposes more animals to suffering due to mis-shooting or killing during the breeding season. We urge the Scottish Government to step in and give them legal protection as a matter of urgency, before the next season’s kits are born.”

Summary information on the shot beavers was obtained by Scottish Green MSP Alison Johnstone, the deputy convener of Holyrood’s cross-party group on animal welfare. “Animal cruelty is simply wrong, and there can be no excuses for inhumane killing of wild creatures in 21st century Scotland,” she said.

“Scottish ministers need to get off the fence, accept that beavers have a positive role to play in terms of biodiversity, and that they deserve legal protection. The poor treatment of these amazing creatures will shock many people.”

Concerns have also been expressed by the Scottish Wildlife Trust, and by SNH. “We share welfare concerns about beavers shot with inappropriate firearms and ammunition, and those with dependent young,” said SNH’s director of operations Nick Halfhide.

“We have asked land managers not to shoot beavers in Tayside but instead seek advice from us on mitigation, such as protecting trees and discouraging dam building. However, if they choose to use lethal control, we have offered them advice on how to do so humanely.”

The National Farmers Union in Scotland said the failure to remove illegal beavers from Tayside had pushed the responsibility onto farmers. Farmers had been refused permission to trap the animals, and mitigation methods had not proved effective.

“They have therefore had to carry out legal lethal control as a last resort in order to protect very productive farmland from damage due to drainage systems being blocked by beaver activity,” said the union’s deputy director Andrew Bauer.

“In the vast majority of cases, those beavers that have been shot have been killed humanely. In a very small number of cases, beavers may not have been killed humanely – a sad outcome and one that no farmer would wish for.”

Scottish Land and Estates, which represents landowners, criticised the Scottish Government for failing to issue advice on controlling beavers. “Our own advice to farmers and landowners has been to control only where it is necessary, to look at all options and, if lethal control is the only reasonable option, to carry this out in the most humane way possible,” said the group’s Ann Gray.

Following a trial with beavers at Knapdale in Argyll, Scottish ministers originally promised a decision in 2015 on whether or not to allow their reintroduction in Scotland. But the decision, which could give the Tayside beavers legal protection, has still to be made, and may be months away.

The environment minister, Aileen McLeod, “will be taking time to consider the issue carefully and listen to the views of stakeholders before making a decision on the future of beavers in Scotland,” said a Scottish Government spokeswoman.

“There is currently no legal protection for beavers in Scotland. We are aware that farmers on Tayside are experiencing issues with beavers and encourage land managers to consult with SNH on mitigation measures rather than resort to lethal control.”