A prominent animal rights campaigner has been given a six-month suspended jail sentence for breaking an injunction designed to prevent him from disrupting the trial badger culls.

Jay Tiernan, a leading campaigner in the Stop the Cull movement which backs direct action, was also ordered to pay costs that could total as much as £55,000.

High court judge, Sir David Eady, ruled that Tiernan was in contempt of court for nine breaches of an injunction secured by the farmers union the NFU, which championed the trial culls in Somerset and Gloucestershire.

The breaches ranged from Tiernan filming someone involved in the cull to him protesting outside an NFU office wearing a T-shirt reading: “FCK NFU”, and failing to pass on details of the injunction to other protesters.

Tiernan, who is based in the south-west of England, said he was “flabbergasted’ that he had received such a heavy sentence. “I was in the British army when I was a teenager,” he said. “I believed that England was a place where we had rights and freedom. All I have done is try to illuminate something I believe is wrong.

“This sort of sentence doesn’t just affect animal rights campaigners. It affects everyone. When people can’t speak out, it’s a perversion and corruption of laws and rights.”

Tiernan said he would try to raise the money to pay court costs via crowd-funding and would write a book on his struggle against the badger cull trials in Somerset and Gloucestershire.

He said the sentence, which is suspended for two years, would make him change his modus operandi but would not prevent him fighting any future culls. A new tactic in the coming months by Stop the Cull will be to target marginal seats held by the Conservatives. “They are trying to stop me speaking up. They won’t succeed.”

The NFU said it believed the sentence would send a warning to other protesters.

NFU president Meurig Raymond said: “We welcome the court’s finding of contempt in this case, and the sentence imposed for breaching the terms of the high court injunction. This sends a clear message that the injunction has teeth and such action will not be tolerated.

“We recognise that the badger cull is controversial and we have never sought to stop lawful protest. But we cannot condone actions designed to harass, intimidate and threaten farmers and others protected by the injunction. That is why we applied for the injunction in the first place, and why we felt compelled to bring these proceedings against Mr Tiernan for breaching the terms of the injunction.

“Mr Tiernan has publicly said he doesn’t care if farmers or people living in the cull areas are frightened by the actions of activists if it meant people pulled out of the cull or didn’t sign up for it.

“He has endorsed the use of military-style tactics to disrupt operations, and treated the high court injunction, which was granted to stop exactly this kind of behaviour, with utter contempt.

“Today’s finding sends a clear message that this kind of behaviour against people carrying out a legal and licensed activity as part of a government policy to help deal with bovine TB will not be tolerated.”

At the high court in London Sir David said the cull was undoubtedly controversial but a challenge to its legality had been rejected by the courts.

He said that Tiernan, who had chosen to represent himself, was a man of good character. The breaches did not involve any violence or damage to property and Tiernan had apologised in court to those concerned.

“On the other hand, the breaches were clearly deliberate and defiant of the court’s orders,” said the judge. One breach – that of failing to notify activists of the terms of the order – was still continuing, he added.

“The offences certainly reach the custody threshold, but there is room for the intermediate step of suspending the sentence,” he said.

Writing for the Guardian last year Tiernan said people’s reasons for fighting the cull varied but he added: “For me it is simple: as a vegan I believe all animals should be wild. The killing of badgers to somehow ‘save’ dairy and beef cows is perverse. The dairy cow already leads a short, miserable life of as few as five years compared with a natural life span of 25.”