At present, H5 viruses can only infect one of the two main types of cell in the mouth and nose, a type of cell known as a ciliated cell.
In order for H5 to transmit from human to human, it would need to mutate to be able to infect the other, non-ciliated type of cell as well.
Lead researcher Professor Wendy Barclay, from the Division of Investigative Science at Imperial College London, said: 'Our research suggests that it is less likely than we thought that H5N1 will cause a pandemic, because it is far harder for it to infect the right cells.
'The odds of it undergoing the kind of double mutation that would be needed are extremely low. However, viruses mutate all the time, so we should not be complacent.'