The licences given to scientists that allow them to carry out experiments on animals are being mislabelled, according to a High Court ruling.
Buav complained about experiments on marmosets
The British Union for the Abolition of Vivisection (Buav) claimed that the licences underplayed the severity of suffering that the animals experienced.
The High Court found for Buav on this ground, but rejected three other claims brought by the campaign group.
The Home Office has been granted leave to appeal against the decision.
If it is unsuccessful, the government will have to re-examine the way it classifies the severity of lab animal tests.
The ruling followed a judicial review that took place earlier this week.
It was prompted by a 10-month investigation carried out by Buav at Cambridge University in 2000-2001.
The campaigning group said that experiments that had been taking place on marmosets were causing far more suffering than their licence implied.
The Home Office grants licences for all experiments carried out in the UK that involve animals.
Each licence is categorised as "mild", "moderate", "substantial" or "unclassified" (meaning the animal is under anaesthetic). This is based on the likely experience of the "average" animal in the experiment.
The Home Office issues licences for animal research
The licence for the marmoset research was classified as moderate, but Buav argued that the experiments should have been classed as severe, leading the group to call for a judicial review, stating that the Home Office was failing to ensure animal suffering was kept to a minimum.
The High Court ruled that Buav was correct in stating that the government was failing to correctly determine the severity limits for animal experiments.
However, it rejected three other grounds the campaigning group put forward: on the fact that animal death should be labelled as an "adverse effect", a technical notice that failed to follow correct legislative procedure, and failings on post-operative care.
'Misleading the public'
Buav chief executive Michelle Thew said: "We have proven that the government misleads the public and Parliament about the severity of animal experiments licensed in the UK.
"The government can no longer pretend it has the strictest regulation of animal experiments in the world. This case demonstrates it has ridden roughshod over the public's trust in this matter."
A Home Office spokesperson said: "The Home Office is obviously disappointed not to have successfully defended all of the grounds of claim, and has been granted leave to appeal against the decisions in the instances where the judge had found in favour of Buav.
"The Home Office believes it has been rigorous in applying the strict criteria of the Animals (Scientific Procedures) Act 1986 with a view to making proper provision for the protection of animals used for experimental and other scientific purposes."
This week, the Home Office announced its official figures on animal tests.
A total of three million procedures were carried out on animals in 2006, a rise of 4% on the previous year.
Scientists said tests were necessary to help cure life-threatening diseases; however antivivisectionists said the rise was too high.
In 2003, the Chief Inspector of the Animals (Scientific Procedures) Inspectorate rejected the notion that Cambridge University had breached its licence when it considered Buav's complaints.
The inspector's inquiry found that the severity levels of the procedures had been correctly assigned; and concluded that "nothing seriously untoward has been discovered about the licensing and running of these projects".