In 2003, BAE Systems - Europe's largest arms company - was accused of spying on a small group of peace campaigners. Mark Thomas refused to believe that his trusted friend and fellow activist Martin Hogbin could possibly have any involvement in the story. But then the doubts began to set in ...

If I were to count the cuts that killed my friendship with a man called Martin Hogbin then the thousandth came within a solitary line of a legal document. This document, dated October 2007, had a dull, dry title: "A Consent Order". And when I read it, years of trust and love slipped away.

But first I had better go back to the beginning.

A sizeable chunk of my work, be it writing, performing or making TV shows, has been about the arms trade, and I met Martin shortly after he joined the peace group I was involved with, the Campaign Against Arms Trade (CAAT). Martin was 45 when he joined CAAT as a volunteer in 1997; in 2000, he became a member of staff as the group's national campaign and events coordinator.

We hit it off: Martin was a breath of fresh air in a world that can sometimes become a tad pious and self-congratulatory. Unlike the mass of anti-arms-trade activists, he played golf and wore Pringle sweaters - you don't tend to see many Pringle sweaters on protests against international merchants of death.

I wasn't the only one to like and trust him, and to be impressed by his work. One of those close to Martin was Steve, who says now that, "for the younger activists, Martin was like a father figure". Martin always seemed to be the last one to leave the police station if a protester was arrested. He always made sure they got home safely - and normally with a drink inside them. He was rude, warm and had a wonderful sense of mischief. He would often come along to my live stand-up shows to help run the stalls, handing out leaflets and flogging books. I would sometimes even refer to him in the shows.

This is one of the stories I told about him. I phoned him one morning to hear his Kent twang bark: "Can't talk, I'm chained to a petrol pump!"

"What?"

"Protest to shut down Esso stations!" he replied.

Whether the cause was climate change or Iraq mattered less than the vision of his ruddy frame strapped to a pump.

"What have the cops done?" I giggled.

"Two of them came over and asked us how long we would be here. I said probably all day. They said, 'All right then, as long as you don't do anything illegal, we'll leave you to it,' and then fucked off!"

"Bloody hell, mate! They could have had you on aggravated trespass."

"I know, I know, but I thought if they weren't going to say anything, neither was I!"

There was a pause before he continued in a matter-of-fact tone: "They did get a bit narky when I tried to light a fag."

Above all else, Martin would always turn up and lend a hand. He seemed to be everywhere: getting kicked out of a company annual general meeting, helping to run a mock fire sale of the Iraqi national bank in the City, dressed as a devil on May Day or organising press conferences at the start of the London Arms Fair. We were friends; I knew his family. He became an integral part of my life.

Then, in September 2003, the Sunday Times exposed a "spy network" run by a woman called Evelyn Le Chêne on behalf of BAE Systems, the giant, multinational arms manufacturer. The story claimed that Le Chêne had a database of more than 148,000 names and addresses of activists, peace campaigners, environmentalists and union members, and that she was running spies who posed as activists to obtain confidential information from pressure groups. According to the story, reports on CAAT were at one point being sent daily to BAE's security group from within the organisation.

Martin phoned that morning: "Fucking hell! Have you seen the papers? There's a spy! Who do you reckon it is?"

"I've got no idea, mate, no idea," I replied.

A week later, however, Martin was suspended from CAAT. An inspection of his computer by staff at the CAAT office had shown he had forwarded emails to a strange email address that no one recognised, with no surname or company name, and he had fallen under suspicion.

His closest friends were furious: not at Martin, but at CAAT. "How could they get it so wrong?" we thought. "How could they think Martin was a spy? Martin is our mate and a great campaigner."

But amid the bar-room bluster lurked a few tiny doubts. I phoned Martin during the first days of the furore.

"You have got to take legal action," I said. "You have been slandered. Take them on."

"Nah, fuck 'em," he rasped back.

"You have to, Martin. Is it the money? We can do benefits - we can raise the money." I wanted him to fight, to prove them wrong.

"Nah, it ain't worth it, if that's what they want to believe, there's nothing I can do about it. Fuck 'em."

I couldn't understand why he didn't want to fight the allegations. Later, in the same phone call, I asked him directly.

"Martin, did you do it?"

"Fuck off! Course I didn't."

"I have to ask, you understand."

"Yeah, yeah, I know."

I took his word for it. Martin had said the accusations were bollocks, so even to look at the file of evidence people said they had on him would be to suspect a friend and that would be an act of treachery on my part. For more than a year, in fact, I defended him and once again, when it was time to tour with my stand-up show, Martin came along. Touring the country, sharing hotel rooms and kipping on the floor in a sleeping bag, Martin helped raise thousands of pounds that funded anti-arms-trade groups and trade unionists visits to Colombia.

But still I had doubts. There were the logical worries, such as: why had he not gone to the CAAT inquiry to clear his name? And there were the instinctive: had his voice sounded weird when he asked: "Who do you reckon it is then?"

The questions never left me. So in 2005 I climbed the narrow stairs to an empty room at the top of CAAT offices in London and sat alone at a wooden table reading the confidential and internal emails Martin had forwarded to a mystery address while working at CAAT. He had always admitted forwarding them, insisting they were to go to an ex-CAAT volunteer. How had they been sent to this mystery address, then - an address unrelated to the ex-volunteer?

Martin has said it was by mistake. But when I looked at the file, I wondered how anyone could make this many "mistakes". I was shaking my head - I don't know if it was an attempt to clear it or a register of disbelief. There were hundreds of emails sent "by mistake". And slowly I became aware that I actually wanted to vomit with the fear that my friend might be a spy.

Shortly afterwards, CAAT revealed that the government's independent information commissioner, Richard Thomas, had investigated the case and found that "a former member" had been forwarding information to an email at a company with links to Le Chêne. He refused to name Martin, stating there was insufficient evidence to do so.

Then last year, CAAT - alongside the environmental and human rights NGO, the Corner House - opted to bring a judicial review of the Serious Fraud Office's decision to drop the investigation into the allegations of bribery between BAE Systems and Saudi Arabia. A month later, in January this year and out of the blue, BAE Systems lawyers contacted CAAT's lawyers and said words to the effect of: "Terribly sorry, old bean, but we appear to have your confidential legal strategy for the judicial review." For non-legal laymen, having possession of the other side's legal work is considered exceptionally bad form, akin to a doctor groping a patient. It's the kind of thing that can get lawyers kicked out of their profession.

The peaceniks at CAAT, not unreasonably, wanted to know how a multinational arms dealer had come by their confidential documents. The company refused to tell them. So CAAT took BAE Systems to court, and there the company was forced to admit that the document had been sent to them, unsolicited, by Paul Mercer, whose company, LigneDeux Associates, was paid by BAE Systems to provide "media and internet monitoring" on CAAT. In essence, they admitted that they had been paying for CAAT to be spied on - an extraordinary admission. Normally, campaigners' tales of being infiltrated by corporations are seen as the imaginings of paranoid conspiracy theorists. The company's admission has changed that. (Mercer claims CAAT's confidential documents were sent to him anonymously in a brown paper envelope. CAAT are continuing legal proceedings against him.)

What makes this even more unprecedented, though, is the company's legal promise not to spy on CAAT in the future. BAE Systems has undertaken "not to intercept by any unlawful means ... [and] not to solicit, voluntarily receive or procure any confidential communication or document" belonging to CAAT. The big picture is that a multimillion-pound arms firm has been humiliated, it has been caught and forced to admit to paying for spying on a peace group comprised primarily of students and Quakers, and has promised not to do it again.

And in the corner of the big picture is my friendship with Martin. Amid all the legalese, in that document marked "A Consent Order", BAE Systems admitted to hiring two people in particular, Paul Mercer and Evelyn Le Chêne. Those were the three words, the name "Evelyn Le Chêne" - they were the 1,000th cut. Le Chêne was a spy organiser living in Kent. Martin Hogbin, my trusted friend, was passing information "by mistake" to a company linked to her. Those were the facts.

Now, four years after the allegations against Martin emerged, I stand on the platform at Paddock Wood waiting for the Maidstone connection. I am heading to Martin's home. I want him to tell me the truth.

Some CAAT supporters believe Martin joined the group with the aim, right from the start, of passing on information - a view I find strangely comforting. This interpretation of events means he befriended us to do his job, and get information, ergo there was no betrayal, as there was no real friendship. But life is messier than that. "It's not black and white, Mark," says Em. She has been one of the key organisers for the protests and direct action against the arms fair in London; Martin Hogbin is the godfather of her son. "I have not spoken to him in nearly two years," she says. "But whatever happened, there were moments in our friendship that are genuine."

Another activist, Gideon, was particularly good friends with Hogbin. In 1999, they spent an afternoon in the Houses of Parliament "dungeon" after they hurled photocopied money covered in fake blood at MPs from the public gallery. "To this day I would still call him a mate," says Gideon, even though he too suspects Martin. "I don't think he did it out of sympathy to the arms dealers," he says. "Maybe it was power."

As I stand in front of Martin's front door, I realise I am scared. Not of the truth, but scared he will open the door and I'll see him as a friend again. I'm scared I'll let him off the hook. I want to ask, if he was a spy, how much we were worth? How much did he get paid for us? I want to know if it was ideological or if it was about the money, or just the thrill of betrayal? Waiting on the concrete patch by his front door, I run through the questions one more time, determined rational fact should triumph over latent affection.

Martin's wife opens the door. "Mark!" she blurts out. "Come in, come in."

"Is Martin in?" I struggle to keep it business-like.

"No, he is at work. Come on in."

There is barely room for us to keep a proper distance as we stand facing each other.

"I'm writing about Martin," I say. "He needs to read this letter." I hand her my letter, in which I outline what I'm going to say about him in this article and ask him to respond. She looks pained, as if expecting the worst.

"I wish this was under different circumstances. I really don't want to drag you into this," I say.

Her hands are held up to her neck, and in a torn voice she exclaims, "I can't say anything, Mark, I daren't say anything."

Awkward and incapable of offering comfort, I say: "How is he?"

"Not good," she says. "He lost all his friends, you, everyone ... We have to make ends meet. You can see ..." She gestures around the tiny house.

If Martin was a spy, his circumstances suggest he got considerably less than 30 pieces of silver. Maybe he is just another victim of the arms trade.

Six days later I get a text from Martin. He accuses me of threatening his wife and then says: "I hope and wish all my old friends health, success and happiness. I miss you all and cherish our achievements and time together. Please do not try to continue to contact me."

And with that text the tale comes to an abrupt end. Martin is the only person who can tell me what he did, if anything, and why, but he doesn't want to talk. I suspect he was a spy and until he decides to speak I can never know the truth about that, or about our relationship. But sometimes when a memory of him emerges unheeded or I catch a glimpse of him in an old photo, I remember that once upon a time a man called Martin Hogbin was my friend ... then I shake my head and get back to work.

The gnat and the elephant

How a tiny peace group irritated Europe's biggest arms company

It seems curious that BAE Systems, Europe's biggest arms company with sales of £13bn a year, should have felt the need to spy on the Campaign Against Arms Trade (CAAT), a small peace group with a budget of less of £250,000 a year. BAE Systems (formerly British Aerospace) is enormously influential within the highest reaches of government; so much so that Robin Cook, the former foreign secretary, once said he never knew Downing Street make any decision that displeased BAE.

CAAT, meanwhile - with its seven paid staff and a limited number of activists - does its best to influence those in power by organising demonstrations and vigils outside the offices of government departments and arms companies. If one is an elephant, the other is a gnat.

But it would appear that gnats can irritate elephants, because in 2003 it was alleged that BAE Systems was paying out £120,000 a year to spy on the peace group. The alleged spy organiser, a woman in her 60s called Evelyn Le Chêne, was understood to have been hired in the mid-1990s, when CAAT was intensifying its campaign against BAE's plans to sell Hawk fighter jets to the repressive regime in Indonesia.

As well as demonstrations, CAAT was writing letters to ministers and MPs, and it was alleged that Le Chêne obtained copies of some of these letters from inside CAAT and passed them to BAE Systems. Her reports were also alleged to contain details of how the activists were seeking to recruit celebrities such as Helen Mirren to their cause.

BAE Systems has in the past refused to comment on the allegations, but it has made clear that it considers anti-arms trade groups such as CAAT a "threat to the company's security". In a recent court document, Mike McGinty, BAE's security director, said: "Some of these groups have, as a result of direct action, caused significant damage to the company's property, put the employees of the company in fear and at risk and disrupted the company's business."

He cited, as examples, activists who caused £1.5m of damage to a warplane and the occupation of an airfield by 60 protestors, which forced it to be closed for a day.

Rob Evans

· Mark Thomas performs More Adventures in Serious Organised Crime at The Venue, Leicester Square, London WC2 until December 15 (except December 9). Go to seetickets.com or call 0870 264 3333. For more information on Thomas's work, go to markthomasinfo.com