Johnson, who chaired a meeting of the government's emergency committee on Monday (November 15), confirmed in a news conference that the Joint Terrorism Analysis Centre (JTAC) had decided to increase the country's threat level to the second highest tier.
The "severe" rating means JTAC deem an attack as "highly likely."
British police said earlier on Monday that the Liverpool blast killed one passenger, who is believed to have made the explosive device and carried it into the taxi.
Police said they believed they knew the identity of the passenger but could not disclose it.
The explosion engulfed the taxi in flames outside Liverpool Women's Hospital on Sunday (November 14), just before a Remembrance Day service to commemorate war dead commenced at nearby Liverpool Cathedral.