Laboratory animals were used in more than four million procedures in the UK last year, according to new figures described as "shocking" by anti-vivisection campaigners.
The annual animal testing report from Britain's Home Office also revealed a 19% increase in experiments involving Old World monkeys since 2013, partly due to the development of "smart" drugs.
Since new European rules were introduced in 2014, only completed animal testing procedures have been recorded.
Procedures are also now rated according to the amount of suffering, distress or lasting harm they cause.
In 2015, a total of 4.14 million procedures were completed, about half of which involved actual experiments. The other half related to the creation or breeding of genetically modified animals that were not used in further tests.
Because of the change in data collection, figures for 2014 were likely to have been "artificially low", said the Home Office - which regulates animal testing.
For this reason, the latest statistics were compared with those for 2013, before the new rules came in. Between 2013 and last year, the total number of recorded procedures was said to have risen by 1%.
Of the 2.08 million experimental procedures carried out, 6% (123,000) were categorised as "severe" in terms of suffering. This proportion had fallen by 2% since 2014.
Almost a quarter of experiments (502,000) fell into the "moderate" category, which includes non life-threatening surgery conducted under general anaesthesia.
Specially protected species - dogs, cats, horses and non-human primates - were used in 0.8% of experimental procedures, birds in 7%, rats in 12% and fish in 14%.
More than a million experiments involved mice, accounting for 61% of laboratory tests.
Old and New World Monkeys were used in 3,600 procedures, dogs in 4,600 and cats in 210.
Anti-vivisection group Cruelty Free International said the figures reflected "unacceptable levels of suffering" permitted by the British Government.
Michelle Thew, chief executive of the animal rights group - formerly the British Union for the Abolition of Vivisection (BUAV) - said: "The public will find the increase in animal suffering in British laboratories shocking and sickening.
"The UK should be leading the way in reducing animal testing, yet we remain one of the world's largest users of animals in experiments.
"This lack of progress is completely unacceptable, and with recent changes in Government, it is disappointing that there is as yet no minister responsible for animal experiments."
People for the Ethical Treatment of Animals (Peta) described the statistics as "staggering", and the group Animal Aid said they were a "scandal".
A sharp rise in the use of Old World primates, such as macaques and rhesus monkeys, has occurred since 2011 when there were 2,100 procedures involving the animals.
The figure rose to 2,900 in 2013 and continued to rise to 3,500 last year. The number of procedures using New World Monkeys, such as marmosets, fell from 310 in 2013 to 130 in 2015.
Just over a quarter of primate procedures last year were classified as "moderate" and less than 1% "severe".
Animals such as monkeys and dogs may be used in tests because of regulations which stipulate that all medicines must be administered to two non-human species before being given to humans.
Advances in the field of targeted "biologic" drugs, which include a number of new cancer therapies, have also led to an increase in primate use.
Many of the drugs are based on antibodies generated by the immune systems of monkeys.
Speaking to journalists, Home Office chief inspector Sue Houlton said: "One reason they (monkeys) have become more relevant is because of the use of novel compounds, biologicals, which are antibody based."
Professor Roger Lemon, from the Institute of Neurology at University College London, said work involving primates was vital to tackling human brain disorders.
"It's the cornerstone of a lot of our understanding of how the brain works and how brain diseases occur," he said.