A fourth person has admitted conspiracy to blackmail after the grave of a relative of guinea pig farm owners was desecrated in Staffordshire.
The remains of Gladys Hammond were removed from her grave
Josephine Mayo, 38, of Birmingham, pleaded guilty to the charge at Nottingham Crown Court.
The remains of a farm owner's relative, Gladys Hammond, were taken from a local grave in October 2004 during a six-year campaign by animal rights protestors.
Mayo, and three men who pleaded guilty on Monday, will be sentenced later.
Jon Ablewhite, of Hawley St, Levenshulme, Manchester, Kerry Whitburn, of Summer Road, Edgbaston, Birmingham and John Smith, of Leicester Street, Wolverhampton, all changed their plea during a preparatory hearing for the trial at Nottingham Crown Court on Monday.
Their charges relate to a campaign which took place between September 1999 and September 2005.
Staffordshire Police said the campaign includes the desecration of Mrs Hammond's grave in October 2004.
The three men and Ms Mayo, of Spring Bank Road, Edgbaston, will appear before a full hearing on 11 and 12 May.
Mayo was released on conditional bail on Tuesday.
The David Hall and Partners' Darley Oaks Farm, in Newchurch, was involved in breeding guinea pigs used in bio-medical research.
In January the family said the farm had closed and its certificate, allowing the breeding and supply of animals for medical research, had been returned to the Home Office.
The graveyard at Yoxall, scene of the desecration
The statement said it was hoped news of the farm's closure would prompt the return of Mrs Hammond's remains.
She was the mother-in-law of the farm's part-owner Christopher Hall.
After the case, Chief Crown Prosecutor for Staffordshire, Harry Ireland, said: "The strength of this prosecution gave the accused no option but to plead guilty.
"These people orchestrated a long running and unpleasant campaign to cause fear and suffering to anyone connected with Darley Oaks farm.
"Their actions went well beyond lawful protesting and persuading, they threatened death and violence, damaged property and used explosive devices."
Mr Ireland added that the prosecution could not prove the four actually physically stole the body of Gladys Hammond, but they admitted using the theft as part of their campaign.
The whereabouts of Mrs Hammond's body remains unknown.
He added: "Their forceful and illegal campaign has been met with an appropriately forceful prosecution.
"These guilty pleas reflect the criminal justice system's determination that communities should be allowed to go about their business free of fear and intimidation."