A COAL-GAS company involving the UK's biggest private landowner, the Duke of Buccleuch, has been accused of "bullying" after it slapped a legal gag on a lone environmental campaigner.

Lawyers acting for Five-Quarter, a Newcastle-based firm that wants to exploit gas in undersea coal seams, have threatened to sue an Ayrshire business analyst for "malicious falsehood". Scottish peer Buccleuch has an 8% stake in the company, and has a director on its board.

Mel Kelly wrote a 42-page report entitled Theft Of Austerity Britain's Coal on what she sees as the danger of underground coal gasification. It criticises Five-Quarter, its directors and other firms, and has been circulated to politicians and the media.

On May 8, she got an email from Newcastle law firm Muckle telling her to "cease and desist" distributing her report to avoid costly legal action. Such gagging has been termed a SLAPP - a strategic lawsuit against public participation.

Muckle told Kelly to remove any reference to her report from any publications, and provide contact details for the report's recipients, with confirmation in 48 hours that this would be done.

"If this confirmation is not received, our client reserves the right to issue proceedings against you seeking relief for defamation and/or malicious falsehood," the email said, adding: "Our client would also be seeking to recover legal costs and interest. Such costs could be substantial."

Kelly's allegations were "untrue and are likely to cause serious harm to the reputation of Five-Quarter", Muckle said. The risks she highlighted were "isolated incidences and do not involve Five-Quarter", it said. Kelly, from Irvine, has asked Muckle for more time to respond, but called the email "a heavy-handed attempt to intimidate me".

She said: "I will continue campaigning to raise awareness of underground coal gasification, which will likely prove to be just as controversial as fracking, once the public and legislators become fully aware of the many possible consequences for our health, wealth and environment."

Dr Richard Dixon, director of Friends of the Earth Scotland, was threatened with legal action by a coal firm last year over tweets lampooning it. "Five-Quarter's threat to Mel Kelly is at a whole different level of nastiness. Over-the-top legal threats to a local campaigner are nothing but bullying," he said.

Green MSP Alison Johnstone said: "If Five-Quarter was hoping a letter from lawyers would help shut down debate about unconventional fossil-fuel extraction, it was extremely naive and its plan has spectacularly backfired."

In a statement, Five-Quarter said it had not sought or obtained a court order and did not seek to suppress debate. It said the company had tried to answer concerns, and had built its reputation on strong environmental credentials. But the statement added: "Where false statements are published that may cause damage, we will take all necessary steps to protect Five-Quarter and its directors."

A Buccleuch spokesman said: "Five-Quarter's licences are for offshore activity and Buccleuch will not be using these technologies in any of its onshore energy initiatives, particularly in the south of Scotland."

The Sunday Herald reported last year that Buccleuch was working in partnership with other companies to dig an open-cast coal mine and exploit coal-bed methane in the Canonbie area.