By Paul Rincon
BBC News science reporter
The rise in use of genetically modified animals in UK labs shows no signs of abating, government figures reveal.
Modified animals made up 32% of all procedures in 2004 compared with 27% the previous year.
Total experiments showed a marginal increase, of 2.3% to just over 2.85 million - but this is still about half the level it was 30 years ago.
The number of animals used in research was 2.78 million, a rise of 2.1% on 2003 figures.
Home Office minister Andy Burnham said that animal research had led to advances in the treatment of many conditions, such as Parkinson's disease, asthma, kidney disease, schizophrenia and peptic ulcers.
"Where there is no alternative available, we will continue to ensure that the balance between animal welfare and scientific advancement is maintained," he commented.
Normal animals were used in 1,673,000 procedures, a fall of 76,000 (4%) from 2003. Genetically modified animals were used in 914,000 procedures, a rise of 150,000 (19%) from 2003. The regulated use of GM animals has more than quadrupled since 1995.
Colin Blakemore, chief executive of the Medical Research Council (MRC), said the increased use of modified animals - predominantly rodents - was a result of new techniques that were giving important insights into human disease.
About 2.85 million procedures started in 2004
About 40% of all procedures use some form of anaesthesia
'Procedures' includes the breeding of animals for research programmes
About 4,200 procedures are carried out on non-human primates
By adding or knocking out genes in mice, scientists believe they can gain an insight into the molecular flaws in humans that lead to illness.
The British Union for the Abolition of Vivisection (Buav) criticised the government for failing to provide adequate funding into alternatives.
Alistair Currie, campaigns director for Buav, told the BBC News website that although the government had given some funding to alternatives for animal research, it "doesn't really have a strategy...on what we do to get the numbers down."
Adolfo Sansolini, the group's chief executive, said the figures did not "include the 'missing millions' of animals bred for vivisection, but killed as surplus to requirements".
Campaign group Animal Aid has published its own report to coincide with the release of the statistics that brands the use of GM animals a "scientific dead end".
The report's author Dr Jarrod Bailey, a medical scientist at Newcastle University, carried out an analysis of scientific papers and found that, in some 70% of cases where a GM animal is created in the hope of replicating human symptoms, the animal does not perform as expected.
"Animals often don't mirror the human situation in terms of symptoms or pathology," Dr Bailey told the BBC News website.
Dr Bailey pointed out that viable alternatives existed, such as experimenting on human tissue grown in the lab and the use of DNA-based methods such as microarrays to look at gene expression.
Professor Chris Higgins, director of the MRC Clinical Sciences Centre, defended the use of GM animals, saying that their use would lead to more precise approaches to developing treatments.
"We cannot understand how the infective agent causing BSE spreads from the gut when ingested as food, to the brain where it causes vCJD, without studying whole animals. Genetically-modified mice provide the best model for this," he explained.
Professor Blakemore expressed concern that campaigns of harrassment and violence by animal rights extremists could harm the welfare of animals bred for research.
"Nearly all of the animals used in medical research come from designated sources in the UK, where animal welfare legislation is very strict," he explained.
"The closure of breeding farms in the UK as a result of harassment and violence could make it necessary to import animals from countries where regulations are not so tight."
Earlier this year, a farm that has been breeding guinea pigs for medical research for more than 30 years was forced to stop after a campaign of intimidation by animal rights activists.
RSPCA senior scientist Maggy Jennings commented: "Rodents can suffer pain and distress. This is not only as a result of experiments, but also through their inability to express their full range of natural behaviours."
The number of procedures using non-human primates in 2004 was 4,208, down by 12% on the previous year.