"I'll be the Irish-looking guy in the blue coat, " says Martin Ingram, which is hardly helpful. But there he is, leaning against a pillar, of medium build with dark eyes and hair, lookingf well, Irish.

At this weekend's Sinn Fein ardfheis, British infiltration of the republican movement isn't being debated. It should be, says Ingram: "It's time ordinary republicans stopped being led like sheep and started asking questions.

"At grassroots level, around one in 20 members are British agents. Higher up, it's one in three." He names a well-known figure and exArmy Council member as a spy.

Former RUC Special Branch officers have lodged documents in London which, if genuine, could lead to the man's exposure. Ingram is considering writing his biography. "Short-term, 'X''s cover is safe but, eventually, he'll be outed."

A Sinn Fein west Belfast representative, whose name is circulating among republican grassroots, has been a Special Branch agent since the 1980s, he claims: "It was common knowledge when I served in Northern Ireland."

'Martin Ingram' is a pseudonym, although the government knows his true identity:

he has an Irish passport. He is giving the Sunday Tribune his first full media interview in three years since co-writing the best-selling book, Stakeknife, on IRA internal security head, Freddie Scappaticci.

So why did 'Scap' and Sinn Fein's Denis Donaldson betray their comrades? "Usually, it's for the money. Donaldson would have earned at least 300,000 a year, taxfree. It's easier to recruit in cities. Belfast and Derry are full of big-time Charlies.

South Armagh was harder . . .the IRA there was built around families."

It's less costly for the British if 'outed' agents break their security links and reach a deal with Sinn Fein, as 'Scap' and Donaldson did.

"Relocating an agent and his family costs up to 1.5m.

"A house must be bought for him, a new identity created, and usually a business set up. Most agents aren't suited to business. When they fail, they're back at the trough looking for more money."

Ingram (43) served in the controversial Force Research Unit (FRU) for eight years.

He left the army after marrying a Co Donegal nurse.

Her family's republican connections meant he could no longer work in a sensitive position in the North.

He has two daughters . . .

"one speaks fluent Irish, I've just left her playing the tin whistle." He is self-employed:

"neither a multi-millionaire, nor poor". He has passed Gerry Adams on Co Donegal streets, and greeted McGuinness in Irish during a radio phone-in programme.

Ingram makes no informer allegations about Adams but alleges his agenda . . . "to defeat the IRA" . . . was the same as British intelligence's "and that this was why they saved his life twice". Stopping a 1984 loyalist assassination attempt would have compromised Brian Nelson, Britain's most senior UDA agent, so instead the British doctored the bullets, ensuring they weren't lethal. In 1987, they thwarted the planting of a limpet mine on Adams's car.

Provisional leaders accuse Ingram of mischief-making to cause internal dissension.

"They never questioned my motivation when I spoke about the British state murdering republicans or when I said loyalists, at leadership level, were 100% infiltrated.

"I've helped republicans on collusion matters. Solicitors for Danny Morrison and the Finucanes asked for meetings. The Andersonstown News published an article by me. The Provos smear me now because what I'm saying is uncomfortable for them."

How does he view leading republicans? "An aura has been created around Martin McGuinness's IRA record . . .

it's bollocks. He never did much as an operational volunteer and, under his leadership, Derry PIRA was riddled with touts." Pat Doherty is "a nothing";

Mitchel McLaughlin, "a teddy bear".

Former Real IRA leader, Mickey McKevitt, "an unreconstructed, serious f***ing terrorist". Slab Murphy is ruthless: "If he thought 'X' was a tout, he'd have him killed. So, thankfully, he doesn't believe me." Although Murphy's desire to "preserve his business empire" has blunted his militancy.

Gerry Adams is "a boring f****r . . . I'd rather spend a night at bingo with Collette (Adams's wife)." Gerry Kelly is "like me, a bit of the lad, he'd be great to go on the piss with."

Ingram, a working-class Leeds United supporter, was in army training during the 1981 hunger-strike. Recruits bought burgers and phoned Sinn Fein "to taunt them when eating Big Macs."

The diplomatic telephone directory was availed of to ring Russian embassies around the world "to take the piss". It was juvenile but "bored squaddies do anything".

He didn't wanted to be posted to Germany, "too boring". Cyprus was his first choice . . . "sun and shagging" . . . but he didn't mind Northern Ireland, "it was exciting, you could make a name for yourself".

Once there, he increasingly sympathised with nationalists: "Protestants seemed very bigoted. We called the RUC 'the Stasi'. We were out to defeat the IRA but they were soldiers like us. Seamus McElwaine and Jim Lynagh (shot dead by the SAS) were brave men. I was completely opposed to terrorism but I had to admire them."

He liked some informers such as 'Busty Brenda' from Co Fermanagh who slept with an IRA man while FRU bugged his house. Others were "money-grabbing, despicable people". FRU paid some informers' telephone and electricity bills. A spy received a set wage, plus a fee for every meeting. "Some would ask for three meetings a week even if they had f*** all to tell because they wanted money. If you liked them, you let them away with it."

FRU handlers were wellpaid. "Our housing and food costs were covered. When we went drinking, we kept receipts. Everybody fiddled their expenses. Girlfriends were taken to dinner on FRU.

If you wanted a book or CD, you said the tout wanted it.

"FRU paid for handlers to socialise with agents. If there was a Man United/Liverpool match, you'd suggest buying tickets for a weekend of social bonding. If the tout said he wasn't interested in football you'd say, 'You are now!'" An English businessman in Co Monaghan who informed on his IRA employees received no payment.

Indeed, he would take FRU to Ballymena's Adair Arms Hotel for a night out.

Unlike the RUC, FRU rarely blackmailed anyone into spying, Ingram claims:

"We used carrots, not sticks.

A reluctant agent isn't a good one." Only one in 20 approached agreed to spy. "A Derry Provo taped our offer and played it on radio. We were done good and proper.

We posted him £50 for the craic."

FRU built up personal contact with potential recruits to soften them psychologically for an approach. Republicans were relaxed, and so more susceptible, when holidaying or engaged in leisure pursuits.

Ingram tried to recruit 'U', a Fermanagh IRA leader in her 20s whose husband was in jail. "She went swimming in the local pool. I'd go down one lane as she went up the other. I'd make eye contact.

She was a tasty bit of stuff and flirty.

"After weeks of this, I positioned myself on a vehicle checkpoint as she crossed the Border, removing my helmet so she could see it was me. I bantered with her, gave her a number to ring, and she took it.

"Twenty minutes later, she was back. She told me which phone in which Portakabin in St Angelo's base the number was for. She'd access to someone who could trace that. She was saying, 'you think you're smart boy, but I'm one step ahead'. I was too frightened to drive back to base. I got a helicopter to pick me up."

He wasn't always careful:

"My nickname was 'Slack Jack'. You're meant to do a reccy (reconnaissance) to check an area was safe before going out. Slack Jack would have said 'f*** that', and just go and do it. He'd confidence because he knew he was good at his job."

Fermanagh republicans recall "Super-Brit", a soldier on the NewtownbutlerClones checkpoint, whose long hair and chat made him stand out from his colleagues.

"I dressed and behaved differently so people would say, 'who the f*** is he?' If they're interested in you, it's easier to develop a rapport, and maybe recruit them."

Each FRU operator handled eight agents and co-handled another four. Handlers lived (undercover) in ordinary houses in Protestant areas. Informers were collected by van at an agreed location. They would be interviewed inside the vehicle . . .

which contained a sofa and kettle . . . or at the handler's home. Informers wouldn't see where they were taken to and the houses had no distinguishing internal or external features.

If publicly challenged about his identity, Ingram had a cover story: "I worked for the BBC or BT, anything with a British connection. If you bluff and think on your feet, you get away with it." Rural areas were more difficult to operate in than cities: "Farmers are so nosey, they miss f*** all."

His FRU years were "the best in my life". "We worked around our socialising . . . never did a job on a Friday night. In Enniskillen, you scored as often as you wanted, even with Catholic girls. They called us 'Maggie's boys'."

FRU employed women, including Capt Margaret Walshaw who handled Brian Nelson.

"Mags was sex on a stick. I tried my hand there but didn't get anywhere. She was the best female operator I worked with . . . single-minded, ruthless and, by God, could she be a bitch!"

Ingram claims at one stage during questioning by the then Metropolitan Police Commissioner, Sir John Stevens, in his collusion inquiry, Walshaw retorted:

"Why don't you go write a f*****g parking ticket?"

Ingram says he was told this by Stevens' detectives.

FRU drink-driving was common: "I fell asleep driving back drunk to Thiepval Barracks. The car hit the kerb, overturned, bounced along the road and we landed in a field with my mate covered in Chinese takeaway.

The police arrived. We were taken to hospital, but never charged.

"Another night, four of us were naked in a car in Fermanagh coming back from a party. The driver was plastered. Police stopped us on the Lisnaskea Road. We said we were squaddies, and were waved through and told to be careful." He admits such behaviour was "reckless and put lives at risk" but claims it was "a way of relieving tension for young men in their 20s in a conflict."

Ingram says his motivation for whistle-blowing is to "tell the truth to the innocent victims who deserve it." He sees himself as "a sniper shooting down bullshit from whomever, f*****g off, then coming back again". "Slan anois!" he says as he heads into the night.