A judge held that the Home Office was acting unlawfully in licensing invasive brain experiments on marmosets at Cambridge University on the basis that "moderate" rather than "substantial" suffering was likely to be caused.

Mr Justice Mitting said the Home Office was relying on expert advice which understated the extent of the "pain, suffering, distress or lasting harm" that could be caused to the animals.

Government lawyers were granted leave to appeal against the finding.

The judge rejected other claims by the British Union for the Abolition of Vivisection (BUAV), including an allegation that the staffing and monitoring arrangements for animals at the Cambridge research laboratories were inadequate.

The BUAV said the Government had been found guilty of "turning a blind eye to substantial suffering of animals in Home Office-licensed experiments and consequently misleading the public over the extent of animal suffering in UK laboratories".

The court case was brought following a 10-month undercover investigation by the BUAV at a Cambridge University neuroscience lab during 2000 and 2001. The investigation revealed that the Home Office had assigned a "moderate" suffering category to experiments which included highly invasive procedures such as removing the top of a marmoset's head and part of the brain to induce strokes.

Guidelines state that any procedure which "may lead to a major departure from the animal's usual state of health and wellbeing" must be categorised as "substantial".