From the BBC's science correspondent:
There have been changes to the spike protein - this is the key the virus uses to unlock the doorway to our body's cells.
One mutation called N501Y alters the most important part of the spike, known as the "receptor-binding domain".
This is where the spike makes first contact with the surface of our body's cells. Any changes that make it easier for the virus to get inside are likely to give it an edge.
"It looks and smells like an important adaptation," said Prof Loman.
The other mutation - a H69/V70 deletion, in which a small part of the spike is removed - has emerged several times before, including famously in infected mink.
Work by Prof Ravi Gupta at the University of Cambridge has suggested this mutation increases infectivity two-fold in lab experiments.
Studies by the same group suggest the deletion makes antibodies from the blood of survivors less effective at attacking the virus.
Prof Gupta told me: "It is rapidly increasing, that's what's worried government, we are worried, most scientists are worried."
In a press conference on Saturday, Chief Science Adviser Patrick Vallance said B.1.1.7 [the new variant], which first appeared in a virus isolated on 20 September, accounted for about 26% of cases in mid-November. “By the week commencing the ninth of December, these figures were much higher,” he said. “So, in London, over 60% of all the cases were the new variant.” Johnson added that the slew of mutations may have increased the virus’ transmissibility by 70%.
Christian Drosten, a virologist at Charité University Hospital in Berlin, says that was premature. “There are too many unknowns to say something like that,” he says. For one thing, the rapid spread of B.1.1.7 might be down to chance. Scientists previously worried that a variant that spread rapidly from Spain to the rest of Europe—confusingly called B.1.177—might be more transmissible, but today they think it is not; it just happened to be carried all over Europe by travelers who spent their holidays in Spain. Something similar might be happening with B.1.1.7, says Angela Rasmussen, a virologist at Georgetown University. Drosten notes that the new mutant also carries a deletion in another viral gene, ORF8, that previous studies suggest might reduce the virus’ ability to spread.
