Steve Barkham, right, spoke of attacking an Asian man

For Jason Gwynne, finding the right tone was everything.

As an infiltrator in the British National Party, he had to convince the party's leaders and activists that he was "one of them", and yet as a journalist he had to ask the kinds of questions which would give him the answers he needed to film.

Play too na�ve and his cover could have been blown. Don't be na�ve enough and risk the success of the entire project.

"The BNP members already understand the grey areas, but I needed to keep asking," he says.

His project began in December 2003 when the BBC was put in touch with a BNP leader, Andy Sykes, who had joined the party after being concerned about asylum seekers but who had quickly become disillusioned with its actions and ideals.

Mr Sykes had been acting as a mole within the BNP since the time of the Bradford riots in 2001, passing on information about its activities to the Trades Union Congress. He agreed to introduce Jason, 33, to other BNP members as an activist and help him to make film evidence of racism within the party.

Undercover operation

The evidence he collected includes one BNP member, Steve Barkham, confessing to a violent assault on an Asian man, and a prospective election candidate admitting to a campaign of pushing dog excrement through the front door of an Asian takeaway.

Another man is seen saying that he wants to kill Asians and attack mosques. Activists are filmed plotting to fire bomb a van being used by to distribute anti-BNP literature.

You fear that your cover could be blown at any point and that months of research would go down the drain and the story lost Jason Gwynne BNP activists admit to race crime

It was not the first undercover operation Jason had worked on - past projects had included documentaries about football hooligans, fugitives and hells angels. But it was the first he had undertaken knowing his cover would be blown at the end of it when the documentary was broadcast.

He spent most of his six months' undercover working in Bradford, where he was based in a house near to Andy Sykes'.

He says: "I was apprehensive about going undercover and it was difficult because I was away from my friends and family. But they knew I was working in difficult circumstances - only my immediate family and my girlfriend knew exactly what I was doing.

"There is also that fear of exposure when you're living undercover - you fear that your cover could be blown at any point and that months of research would go down the drain and the story lost."

Suspicions

Jason came close to exposure several times during the operation and relied on Mr Sykes to help him remain undiscovered. "But if any suspicions were raised they went through Andy, as a senior figure, and he would tell them he was keeping an eye on me and he would tip us off so we knew to be more careful about the kind of questions I was asking."

One further risk that Jason had to avoid was that there should be no chance of him instigating any actions by BNP members, or joining in with any compromising activities, while all the time being seen and heard to agree with the sentiments expressed by other activists - something which left him feeling "very uncomfortable".

He says: "I heard the BNP leader Nick Griffin give a speech inciting racial hatred and the founder, John Tyndall, inciting racial hatred and I heard some awful anti-Semitic remarks."

HAVE YOUR SAY The BNP must be allowed to air their views however much people disagree with them Scott, Leeds, UK Send your response to the documentary

There was a back-up team of producers "lurking around in vans" nearby whenever he was filming under cover. "The team were very close," he says. "If there were any problems I would telephone or text and I kept on my toes so I could run out of the door if there was any hint I had been exposed.

"But it was hard to manage with the hidden cameras. I would have to go to the toilet to try to do things with them and that became very problematic. There would be people beating on the door asking what I was doing in there.

"One time I was in the toilet and BNP Steve Barkham was outside the door saying 'What are you doing in there - are you on the phone to the CIA?' That was pretty nerve-wracking."

Fear of reprisals means the BBC has had to review security for both Jason and Mr Sykes.

"I am fearful, because you never know when you're walking down the street if someone will recognise you. And I am very concerned for Andy," he says. "He wanted to expose these people and prevent the BNP from creating further tensions and problems in the city he loves. He could have walked away but he didn't."