A gang of animal rights fanatics face lengthy jail terms after admitting charges linked to a terror campaign which culminated in the theft of a pensioner's body from her grave.

The victims of the six-year campaign ran a Staffordshire guinea pig farm providing animals for medical research. They eventually abandoned the business in the face of the intimidation.

On Monday, three activists admitted charges in connection with the campaign and yesterday a fourth pleaded guilty to the same charge.

Kerry Whitburn (36), of Summer Road, Edgbaston, Birmingham; John Smith (39), of Leicester Street, Wolverhampton, and John Ablewhite (36), of Hawley Street, Manchester, pleaded guilty on Monday to conspiring to blackmail David Hall and Partners and others connected to Darley Oaks Farm in Newchurch, Staffordshire, between September 1999 and September 2005.

Josephine Mayo (38), of Spring Bank Road, Edgbaston, pleaded guilty yesterday at Nottingham Crown Court to the same charge.

Judge Michael Pert QC described them as "determined and cold-blooded defenders of their perceived cause".

He told Ablewhite, Smith and Whitburn to expect sentences of up to 12 years for their part in the blackmail plot when they are sentenced next month. The maximum sentence is 14 years.

Mayo was told her part warranted no more than six years behind bars.

The campaign of intimidation and harassment against the Hall family and their employees at Darley Oaks began in 1999 as they faced demands to close the site.

The family was subjected to regular demonstrations from protesters and a violent campaign of intimidation and harassment was launched.

In October 2004 the fanatics proved they would stop at nothing to achieve their goal.

The remains of 82-year-old Gladys Hammond - the mother-in-law of one of the brothers who owns the farm - were dug up and removed at night from the graveyard of a church in Staffordshire. In August last year, nearly a year after Mrs Hammond's grave was desecrated, the family decided enough was enough and decided to stop breeding guinea pigs.

Darley Oaks officially reverted to traditional farming in January, but Mrs Hammond's remains have not been returned.

It is understood the prose-cution was unable to prove that Ablewhite, Whitburn, Smith and Mayo were directly responsible for the theft of the pensioner's remains, but could prove they were all aware of it.

Anthony Glass QC, prose-cuting, confirmed the desecration of the grave was "one of the major planks" of the Crown's case against the four, although none has admitted responsibility.

Whitburn, Smith, Ablewhite and Mayo had previously denied the offences and were due to stand trial later this month.

The three men were remanded in custody. Mayo was granted conditional bail to allow for the preparation of pre-sentence reports.

All four defendants will be sentenced at the same court on May 10 and 11.