All ferry companies and all but two airlines are refusing to carry rats, mice and rabbits for scientific testing into Britain after campaigns by animal rights activists, according to an investigation by the BBC and The Times.
Most animals used in British research are bred in Britain, but around 15,000 of those most important for research, including mice bred to model conditions such as Alzheimer's and motor neurone disease, are imported every year.
Paul Drayson, a former science minister, said the blockade was hampering research that could save lives.
"By giving in to the protesters they are inadvertently choking off vital research into some of the most debilitating diseases affecting our society," he wrote in The Times.
Science minister David Willetts said the government was seeking a code of practice on the transport of animals to allow imports to resume.
"I still hope that we can reach a solution that means we carry on having world-class research in Britain," he told BBC radio. "This is standing up for scientific research which is of great benefit to humankind."
Ferry company Stena Line, which stopped importing the animals in January, had been the last firm to transport the animals to Britain by sea, according to The Times.
A spokeswoman for the P&O ferry operator said the company had stopped the imports last summer "as a result of sustained pressure from anti-vivisection groups".
"It's not for us to take a view on the rights and wrongs of what goes on with the animals that come in, but we have to look to the safety of our staff and protect our corporate reputation," the spokeswoman told AFP.
Britain carried out over 3.7 million scientific experiments on animals in 2010, according to government figures released in July.