The number of animals used in experiments has increased recently
More than 500 academics, scientists and doctors in the UK have signed a declaration supporting the use of animals where necessary to advance medical research.
The statement comes 15 years after a similar initiative, and just one day after the closure of a farm's programme to breed guinea pigs for research, following a sustained campaign by protesters.
The animal experimentation debate remains hugely polarised.
How much work involving animals is still carried out in the UK and has it changed since the last declaration of support by scientists?
The number of animals used in experiments dropped away steadily between the 1970s and 1990, but has more or less levelled off in the past 15 years. Since 2001 the number of procedures using animals has risen slightly every year.
The latest figures available show 2.8 million procedures using animals were recorded in 2003 - about half the number carried out in the early 1970s.
The vast majority - 85% - used in tests are specially bred rats, mice and other rodents.
Campaigners criticise the lack of detailed information on animal tests and say scientists are reluctant to share data.
UK ANIMAL PROCEDURES (2003)
About 2.8m new 'procedures' were carried out
'Procedures' includes the breeding of animals for research programmes
Mice are the dominant research tool, followed by rats
About 40% of all procedures use some form of anaesthesia
Non-human primates are used in less than 1% of experiments
Invertebrates like fruit flies and worms are used in research but not protected by British law
While the number of normal animals used in experiments continues to drop, since 1990 there has been an increase in animals bred with genetic modifications or defects, from around 200,000 to more than one million.
This, says the Research Defence Society (RDS), is because with the mapping of the human genome - which began in 1990 - scientists have identified important genes but do not yet know their functions.
The only way to find out what those gene functions are is to "knock them out" (remove) or add them to a mouse, said an RDS spokesperson.
But according to the British Union for the Abolition of Vivisection (Buav), genetic engineering has a "unique capacity to cause immense suffering and harm to animals".
Their campaigners say techniques used are often poorly understood and produce unpredictable results. Animals develop tumours, brain defects, limb and skull deformities, infertility, arthritis, diabetes and other metabolic disorders, they say.
But RDS says the practice is "more precise" in terms of modelling a particular human condition.
A report this year into the ethics of animal testing in the UK - drawn up by scientists, animal rights groups, philosophers and a lawyer - said the true effects of genetic modification could be difficult to assess.
The panel, set up by the Nuffield Council on Bioethics, said more effort should be made to assess and monitor the welfare of animals used in genetic experiments.
WHAT ARE ANIMAL TESTS FOR?
32%: Genetic studies, eg to find the function of a gene or study diseases
31%: Developing ways to treat/prevent diseases, eg multiple sclerosis, plus testing new medicines for safety
31%: Fundamental biological and medical research, eg finding out how the brain works
4%: Safety testing of non-medical products, eg pesticides. No testing of cosmetics since 1998
2%: Developing new methods of diagnosis, eg scanning unborn babies
Home Office 2003/RDS
Earlier this year, the government gave an extra £3m funding to a centre set up for the replacement, refinement and reduction of animals in research - known as the "three R's".
The law governing animal experiments in the UK is widely regarded within the scientific community as one of the strictest in the world, although Buav argues this does "not necessarily equate to better protection for lab animals".
Professor Ian Jackson, of the MRC Human Genetics Unit in Edinburgh, holds a licence to experiment on mice. The unit's purpose is to understand genetic factors involved in human disease and development.
"There may be a burden of administration within the law, but it makes you think about the work you are doing. For every experiment that is licensed you have to state explicitly why the benefits to medical, and veterinary, research outweigh the cost to animal welfare."
Scientists 'may leave'
Every project also has to go through an ethical review, he added.
"Scientists and some animal welfare groups believe it is better the work is done here, under strict regulations, than abroad. Sometimes when you look at experiments done elsewhere in the world you wonder about their particular regulations," he said.
But pharmaceutical companies and research scientists would consider moving their work abroad if animal rights activists made it too difficult to stay in the UK, said Prof Jackson.
The fact that scientists wishing to do stem cell research were coming to the UK from the USA - where there is no government funding for research on new stem cell lines - "proved scientists were mobile," he added.
Meanwhile animal rights groups have sustained their fight against any experiments being carried out on animals, and have claimed some successes.
In July 2004, a construction firm abandoned a contract to build labs at Oxford University after its shares fell 19% in one day when animal rights campaigners wrote to shareholders threatening to publicise their investments.
Earlier that year plans to build a controversial centre for experiments on monkeys were shelved by Cambridge University, who cited security costs as one of the reasons for its decision.
Plans for a primate lab were shelved by Cambridge University
And a campaign waged against vivisection lab Huntingdon Life Sciences, aimed at hitting it in the pocket, has meant a string of companies have severed ties with the firm.
This week, Darley Oaks farm in Staffordshire said it would stop breeding guinea pigs for medical research after action by animal rights activists, including the theft of a body from a grave.
But professor of political sociology at Reading University, Dr PAJ Waddington - who has studied protests - said auditing success and failure of animal rights groups was "far from straightforward".
"Effectiveness is a complex issue - in what respects have they been effective? Closing down a farm breeding guinea pigs is a scalp, but if fur has returned to fashion, for example, then an advance on one front will be offset by a retreat on another.
"Then there is the issue of time span: because a farm appears to go out of business doesn't mean that the business will not continue to be done elsewhere."
A new law in 1986 meant 'procedures' - which has a broader definition - rather than 'experiments' were counted
The Animals (Scientific Procedures) Act 1986 applies to all living vertebrate animals, plus one species of octopus, used in scientific procedures in Britain. Licences to carry out experiments are issued under the Act
Scientists say the law is strict and protects animals while campaigners argue it protects researchers and legalises cruelty