Why is new variant of Coronavirus more dangerous?

Since the arrival of the new variant of Coronavirus, the British government and the health workers are working day in and day out in order to save maximum lives. But the last 24 hours did not bring any good news for the residents. 
The UK registered record of 744 deaths within a day due to deadly Coronavirus.

On the other hand, the spike in the number of Coronavirus cases has been doubled as the new variant is more deadly.