Animal welfare extremists have had some success in their terror campaigns, a leading expert admitted last night.
Without the activists, more scientists would be experimenting on animals in the search for new treatments and cures for disease, said heart specialist Professor John Martin.
Prof Martin said: "Many scientists are still frightened of the threat and many of them will not do experiments because of that.
"If there was a change in that attitude among scientists then there would probably be an increase in the number of animals used. Many people don't do this research because of the threat to themselves and their families by these extremists."
Prof Martin, from the Centre for Cardiovascular Biology & Medicine at University College London, personally welcomed the fact that more animal testing was taking place.
He said: "Am I pleased that numbers have increased? I think that I am. The fact that numbers are increasing means that in this country we are increasing research into how diseases work."
A Home Office report showed that just under 2.9 million animal procedures were performed in 2005 with genetically modified animals, mostly rodents, representing a third of all the animals used in laboratories.
Their numbers had risen by 1 per cent from 2004. However, two thirds of animals in the GM procedures list were only used for breeding, and did not undergo testing.
Experiments in which animals were created bearing genes linked to human disease were at the forefront of medical science, said Prof Martin.
Mice and rats are the standard tools for much of this research. Last year 42,000 more mice were used in the laboratory than the year before, a rise of two per cent.
Prof Martin said he expected the number of mice used to increase further.
Current testing on GM rats could yield revolutionary stem cell treatments for failing hearts in five years, he said.