Seven animal rights activists who blackmailed companies linked to Huntingdon Life Sciences in an attempt to close down the animal testing lab were today jailed for between four and 11 years.

The six-year international conspiracy between 2001 and 2007 targeted firms across the UK and Europe that either supplied or had secondary links with the Cambridge-based company.

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The leaders of the blackmail conspiracy, Gregg Avery, 41, Natasha Avery, 39, and Heather Nicholson, 41, were founder members of Stop Huntingdon Animal Cruelty (Shac) which was a front organisation which organised strategy and attacks on the firms and their staff, often under the badge of the Animal Liberation Front.

They were assisted by computer expert Gavin Medd-Hall, 45, who researched the victims for Shac, and three "foot soldiers", Gerrah Selby, 20, Daniel Wadham, 21, and Daniel Amos, 22.

Sentencing the activists at Winchester Crown Court, Mr Justice Butterfield called the campaign "urban terrorism" and a "relentless, sustained and merciless persecution" which had made the victims lives "a living hell".

The campaign used threats such as claiming that managers of companies which supplied Huntingdon Life Sciences (HLS) were paedophiles.

Homes of staff, and firms, had hoax bomb parcels sent to them or received threatening telephone calls. Threats of violence were also used to force companies to cut links with HLS.

Words like "puppy killer" and "scum" were painted on homes and cars of workers, cars were paint stripped, and used sanitary towels were posted to the victims with a note saying the sender had Aids.

The aim was to target suppliers or any company with a secondary link with HLS, and the campaign would only stop when the company put out a "capitulation statement" saying it would end links.

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Heather Nicholson, from Eversley in Hampshire, received 11 years after she was convicted of conspiracy to blackmail at a trial last year.

Gregg Avery and his wife Natasha, also from Eversley, received nine years because they pleaded guilty to the charge.

Medd-Hall, from Croydon, south London, who was convicted at the same trial as Nicholson, received eight years.

Wadham, from Bromley, south-east London, was sentenced to five years after he was convicted last year.

Selby, from Chiswick, west London, who was convicted of the charge at the same trial as the others, received four years, and Amos from Church Crookham, Hants, who pleaded guilty to conspiracy to blackmail, received four years.

Mr Justice Butterfield said he accepted that the seven had genuine deeply-held beliefs that animal testing was wrong, and had the right to protest against it.

But he told the activists that companies "had the right to conduct vital biomedical research" and "the right to conduct lawful trading".

He said Shac was "a hypocritical scam" which hid its true intention of terrorising companies and their staff who had links with HLS.

"It was a relentless, sustained campaign designed to strike such fear in the minds of employees that the companies would capitulate," he said.

He called the leaders of the conspiracy "lifelong, veteran, fanatical animal rights activists" and said he had little confidence they would not continue their activities when released from prison.

"I expect you will be seen by some as martyrs for a noble cause but that would be misplaced," he told all seven.

"You are not going to prison for expressing your beliefs, you are going to prison because you have committed a serious criminal offence."

During the ten-week trial, the court heard that all seven were involved in the conspiracy, based a bungalow near Hook in Hampshire.

The campaign targeted firms and staff in the UK but also France, Germany, Sweden and Switzerland, with many of the conspirators travelling to the continent to protest.

About 40 companies were victimised and the total cost of damage and increased security cost £12.6 million, not including loss of profits, the court was told.

Gregg and Natasha Avery and Nicholson founded Shac and gave it the strategy and leadership.

Medd-Hall was a computer and research expert who was high up in Shac and who uncovered company links with HLS, so allowing the firms to be targeted.

Wadham joined Shac in 2005 and regularly attended demonstrations against the firms and HLS.

Selby was also a regular activist at demonstrations in the UK and Europe, including a violent protest in Paris. The judge described her involvement as a tragedy and said she was a hard working and decent woman.

Amos was also an activist who had attended demonstrations and was an insider to the conspiracy.

The judge also gave indefinite Anti Social Behaviour Orders (Asbos) to Gregg and Natasha Avery, Nicholson and Medd-Hall, banning them from travelling to the firms targeted in the campaign.

The others received the same Asbos but for five years.

Speaking after the case Detective Chief Inspector Andy Robbins from Kent Police, who led the £4 million inquiry involving five forces, said: "I hope today's sentences provide some comfort and a sense of justice to the individuals and the families who suffered such sustained harassment.

"While rarely causing physical harm, these offenders thrived on the fear they created through threats and intimidation."

Science Minister Lord Drayson added: "Those involved in life-saving medical research make a huge contribution to society. They deserve our thanks, support and protection.

"The UK is a world leader in medical advances and the Government is proud of the pioneering work of our scientists and researchers.

"The use of animals is essential to the success of this vital work. It is also a legal requirement in the development of safe and effective medicines.

"The UK has one of the most rigorous systems in the world to ensure that animal research and testing is only carried out where absolutely necessary and under stringent controls where suffering must be kept to an absolute minimum."

Home Officer Minister Alan Campbell said: "I welcome the conviction of these animal rights extremists.

"The Government will continue to stand up to animal rights extremism and protect those targeted by it, which is why we have under taken a number of robust measures over the past three years to tackle the threat from a small number of animal rights extremists.

"The Government is fully committed to supporting lawful protest but is also clear that criminal acts including campaigns of blackmail, harassment and intimidation will not be tolerated, which is why animal extremists will be subject to the full force of the law."