Harris, formerly of Clarence Road, Gosport, admitted his role in planning attacks on four branches of Barclays Bank in Hampshire including Park Gate and Bishop's Waltham in September 2008.

The banks, targeted for trading in shares for Huntingdon Life Sciences (HLS), were daubed with painted slogans such as 'Scum', 'Murderers' and 'ALF', short for Animal Liberation Front.

A van belonging to a Fedex employee was covered with similar slogans and had its tyres slashed in Wickham the same month.

And a family had its car covered in paint thinner in a case of mistaken identity.

All the targets were believed by activists to be connected to HLS in Cambridgeshire and were part of a wider campaign to close the labs.

Harris pleaded guilty on the basis that he was a member of the group but not the leader.

Det Con Insp Andy Robbins, who led the investigation and now heads the National Domestic Extremism Team, said: 'These were carefully planned criminal offences carried out during the cover of darkness.

'The vast majority of campaigners conduct their campaigns peacefully. However, where an individual uses crime to further their cause, we will investigate and are committed to bringing offenders to justice.'

The Crown says that following the jailing of senior members of Stop Huntingdon Animal Cruelty (SHAC) in 2007, Harris took over the reins of running the campaign group.

The court is allowing a similar charge against Harris' girlfriend, Nicola Tapping, 27, to lie on the file and it will now not be pursued.

Christopher Potter, 20, and Maria Neal, 21, both from Alcester in Warwickshire, admitted the same charge at an earlier hearing in December.

The couple had been staying with Harris and Tapping in Gosport at the time of the attacks. All three are expected be sentenced next Thursday.

Harris is serving four years for admitting conspiracy to blackmail companies and suppliers linked to HLS.

Tapping was jailed for 15 months at the same hearing in October for admitting conspiracy to harm HLS.