A leading corporate intelligence firm infiltrated the worldwide campaign to ban asbestos in a sophisticated and long-running espionage campaign, the high court in London has heard.

Over a period of four years, the court was told, a spy working for K2 Intelligence Ltd masqueraded as a sympathetic documentary maker in order to gather a mass of sensitive material about the leading figures in the campaign, their methods, funding and future plans.

K2 was in turn passing the information to its client, an as yet unnamed corporation based outside the UK that has interests in the asbestos industry.

On Monday the court ordered K2 to identify its client, although the name of the spy – who was paid in excess of £460,000 in salary and expenses – remains a secret. In a witness statement to the court, the spy – currently anonymised as DNT, by order of the high court – said K2 had paid him £336,015 and he had received £130,430 in expenses.

Although the health risks associated with exposure to asbestos have been known for generations, and its use has been banned in the UK and EU since 1999, use of the mineral known as chrysotile – or white asbestos – is still permitted in most countries.

Major producers include Russia, China, Brazil and Kazakhstan, and key importers include Thailand and Vietnam.

A worldwide network of activists, lawyers and physicians who are campaigning for a complete ban on the export of chrysotile have accused the asbestos industry of running misinformation campaigns and bribing government officials, and say it harasses, vilifies and intimidates its critics.

In a case that may shed new light on the activities and methods of corporate intelligence firms, two activists and a lawyer who advised the anti-asbestos campaign are claiming aggravated damages from DNT, K2 and its London-based managing director, Matteo Bigazzi.

K2 will argue that the amount of confidential information that was collected was tiny, and that the purpose of its investigation was not to publish the information but to better understand the workings of the anti-asbestos campaign.

K2 was founded by Jules Kroll, a leading figure in the corporate intelligence industry, and is run by his son and co-founder, Jeremy.

The claimants are alleging breach of confidence, misuse of private information and breach of the Data Protection Act.

Damages are also being sought from DNT, the man accused of spying on them. He has been granted anonymity as the court has heard that he too is worried about the “intimidatory behaviour” of the asbestos industry.

The court heard that Laurie Kazan Allen, the London-based co-ordinator of the International Ban Asbestos Secretariat (IBAS), says she was approached in 2012 by DNT, who purported to be a journalist and film-maker and who claimed to be planning to make a documentary about the dangers of asbestos.

Through Allen he made contact with other activists, including in Thailand, and attended one conference as a delegate.

“They believed they were speaking to a journalist,” Guy Vassall-Adams QC, for the claimants, told the court. “It was a lie. He wasn’t a journalist, he was a spy.”

IBAS learned of DNT’s true intentions after receiving a tip-off from another NGO with which he had become involved, the court has heard.

Among the material that DNT has been obliged to hand over to the claimants’ lawyers since they started proceedings is a strategy document in which he set out a plan, codenamed Project Spring, to infiltrate the network.

The court has heard that the document was written for Bigazzi, whom Vassall-Adams described as DNT’s “handler”.

In the document, DNT wrote that he had decided to approach a magazine called Hazards in the knowledge that members of its staff were likely to put him in touch with IBAS.

“If you read the IBAS website you very quickly detect a degree of (justified) paranoia about the underhand tactics deployed by the asbestos industry to undermine and harass its critics,” he wrote. “If I arrive with a too tailor-made calling card, might I look too good to be true?”

DNT added: “I am confident I can enter this world relatively easily and with a high level of legitimacy and credibility.”

The claimants’ lawyers allege that DNT communicated confidential information to Bigazzi not by sending him emails but by saving emails as drafts in an account to which they both had access.

Some details of DNT’s activities that have emerged in open court cannot be reported. The judge, Mrs Justice Laing, issued an order under the Contempt of Court Act on the grounds that these details were set out in a document that both the claimants and defendants had agreed should remain confidential.

According to a witness statement submitted to the court by Richard Meeran, the claimants’ solicitor from the law firm Leigh Day, once DNT had infiltrated the worldwide network, he went on to target the World Health Organisation and United Nations agencies that have mandates on asbestos.

He was also able to travel to Thailand, where he gathered information about that country’s plans to ban asbestos, including “an extraordinary amount of confidential and sensitive information [about] the internal divisions between different government ministries on the desirability and likelihood of a ban” and the work of local asbestos opponents.

DNT has handed to the claimants’ lawyers 35,000 documents that he gathered in the course of the four years.

Despite the payments DNT was getting from K2, Allen alleges that he persuaded her to give him £6,530 on the basis that he had no other income.

The time sheets that DNT submitted to K2 show that when Allen was being treated in hospital after a heart attack, he made a claim for the time spent writing a letter to her husband.

Meeran’s statement says Allen was devastated to discover she had been betrayed by DNT. “She has gone through feelings of denial, confusion, emotional outrage, emotional upheaval, distress, shock, anguish and anxiety. She questions the validity of the work she has done over the last four years, and the value of her life’s work.

“She has said that she feels like the victim of an intimate, aggressive, invasive attack. She has been having trouble sleeping and suffered from feeling immense shame for being entrapped by DNT and K2.”

Desmond Browne QC, for K2, told the court that anti-asbestos campaigners could also be ruthless, and had been guilty of campaigns of intimidation and criminal damage. He said there was a need “to counter the impression that this is a contest between virtue, which they [the claimants] represent, and vice, which the defendants represent”.

There were highly significant differences between chrysotile and other forms of asbestos, Browne said.

Of the 35,000 documents gathered by DNT, just 650 had been passed to K2, Browne added, and these had informed 20 intelligence reports that were handed to the client.

On Monday, the court ordered K2 to identify its overseas client to the claimants. The firm had argued that it should not be obliged to do this. K2’s lawyers are expected to ask that the client be granted anonymity if it too faces legal action.

The case has been adjourned until January.