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Ex-soldier Simon Nicholson joined the party in 2007, believing leader Griffin's claim that the party was no longer racist and would campaign on local issues.
He was even praised by Griffin when Simon came within 16 votes of overturning a 1,000-vote Labour majority on Cumbria County Council.
But after growing disillusioned with the party's racism, Simon agreed to work with the Sunday Mirror just before and during this General Election campaign. For nearly three months he kept a diary exposing the sickening racist extremism of BNP members and the true face of the party's far-right views.
Simon, 37, got right to the heart of the B N P 's election machine, At one rally in the North West, he watched as Griffin branded women in burkhas "big, black crows".
At a meeting in Cumbria, BNP National Organiser Clive Jefferson hit out at plans for a Muslim school in Burnley, labelling the scheme "a breeding programme".
Other BNP activists criticised mixed marriages and branded an area of London which has a large black community as "C**n City".
Simon also secretly filmed senior figures admitting how Griffin had taken advantage of the generous expenses system for Euro-MPs, bragging that "we play them at their own game".
Simon told the Sunday Mirror: "What I found was a party dominated by thugs who were still obsessed by race - and Griffin was the worst of the lot.
"The British National Party is a shambles and Griffin is telling lies to voters when he says he has the answers to their problems."
A History Of Hatred
Fascist John Tyndall formed the British National Party in 1982 after a split in the farright National Front movement.
The party quickly gained a reputation for racism, anti-semitism and intimidation of opponents but won its first council seat in 1993 in London's Tower Hamlets.
Under Nick Griffin, made leader in 1999, the party has made further gains and last year won two seats in the European Parliament.
In March the BNP was forced by the courts to allow non-whites to join the party.
REVEALED: THEIR SICK WORLD
'Muslims are going to fly the black flag of Islam over Downing St ' - NICK GRIFFIN in Blackburn
Here is a selection of entries from the diary kept by Simon Nicholson during his time in the BNP...
Saturday Feb 13
Drive overnight from Cumbria for "national day of action" in Barking, where "the boss" Nick Griffin is standing against Labour MP Margaret Hodge.
Arrive at hotel and see a couple of Orthodox Jewish men in the street. Regional Organiser Clive Jefferson mutters: "Look at those f***ing Yids." Three of us crammed into tiny room, Clive rants about Muslims gang-raping "our women" and mixed marriages making him sick.
Griffin puts a couple of leaflets in letterboxes but barely speaks to a single local voter. Chaperoned through a few empty streets with two security cars and his bodyguards.
Later 30 members, including Griffin and other ex-National Front members, gather at The Crown pub in Epping Forest.
A party veteran tells us he used to live in Edmonton but moved out after it became "C**n City". Hates Enfield - "they're breeding like rabbits" - and didn't think much of the BNP's target area of East London today: "They'll f***ing blow the place up. I couldn't wait to leave there after what I saw today."
Thursday Feb 18
At a pub in Sandwith, Cumbria. Clive Jefferson claims "teams of militant Asians and Islamics" will be trying to infiltrate the party. But makes clear new constitution allowing ethnic minority members will be phrased carefully so even those who pledge loyalty to the BNP will be outcasts.
When Sikh member Rajinder Singh's name comes up, one member shouts "black b*****d". Another asks: "What are we supposed to call them when they come to meetings? N*****s? They like being called n*****s, don't they?" Clive suggests putting local Muslims "under surveillance" at Friday prayers and claims Burnley has metal detectors because kids have been stabbing each other in race rows.
Has a go at plan for all-girl Muslim school there, falsely claiming it will house 5,000 Muslims. Calls it "a breeding programme".
Later communications officer Martin Wingfield admits Euro-MPs Griffin and Andrew Brons - despite slamming MPs over expenses - are gladly milking the generous EU system for all it's worth.
"We play them at their own game", he says, then reveals BNP support is dropping off a cliff ahead of General Election. He warns: "Suddenly our votes are tailing off. They're going down and we can't understand why."
Weds&Thur Mar 16-17 JUST 10 card-carrying members on big trip to see Griffin at EU parliament, with another 28 of the 50 seats filled by random crowd, mostly blokes picked up around Cumbria - some necking vodka.
Hellish journey. At Brussels hotel a few of our lot spot two dark-skinned men and start banging the window and making monkey noises. On way to parliament, bus stops at traffic lights near a mosque - hard-core members smash their fists against window shouting abuse.
Told that on a recent trip to Brussels a BNP official spent the night in a brothel. We give that a miss. But our group, many of them out of their minds on booze, still causes chaos - reports of naked women running from room to room at 3.30am.
Journey back hellish, drunkenness and couples copping off. One female member hands in her card in disgust a few days later.
Sunday March 28
In charge of Griffin's security at Moorclose Hotel, Workington.
He's massively paranoid, but his security team love all the cloak and dagger stuff. Would usually be hundreds here but only about 20 today. Reflects what people think - they feel cheated. Griffin's abandoned them and gone down South to campaign for a Westminster seat.
Griffin's answer to the scourge of knife crime: "Most of the disaffected youths carrying out knife crime and gun crime in London aren't ours at all - they're criminals from other countries. They can go back there." He claims Muslims "14 times more productive than us" and soon UK, Ireland, France and the Netherlands "will be Islamic".
In Blackburn, Griffin claims burkhas nothing to do with religion: "It's their way of saying, 'We're here, we're outbreeding you, your Government's paying us to take over and we're going to fly the black flag of Islam over your No 10 Downing Street'." Describes women in burkhas as "like big, black crows".
Thursday April 1
Civil war among the leadership. Total chaos. Jefferson says Mark Collett, the publicity director, has been sacked, because he was "a bad influence on the party". National organiser Eddy Butler and staff manager Emma Colgate have also left their posts - Clive getting Butler's job.
Bulletin for key staff later reveals allegations that officials plotted a "palace coup" against Griffin. Then Collett arrested over alleged threats to kill Griffin. All the talk is of a catastrophic split and massive dissatisfaction with Griffin.
Friday April 23
St George's Day in Sandwith - small event in local pub. Apparently there's been a massive cock-up with election leaflets and hundreds might be useless - Jefferson can't make the event because he's been up all night trying to sort it.
He's already in trouble for failing to take part in live web debate, answering questions from local voters.
Campaign up here is in disarray.
Unbelievable. Been asked to take a job at April 28 party's North West office in Wigton as well as standing in the next council elections.
Party is falling apart at the seams and unpopular Griffin struggling to keep it together. Cock-ups left, right and centre and they can't get respectable people to work for them or stand in elections.
No way I could say yes. Regret ever joining them. I want to stand against them next year because now I know how bad they will be for working class people like us.
Source: The Mirror