According to May, earlier in the day she had been updated by the UK police and the security services on the investigation into Monday's attack in Manchester, which had killed 22 people and left dozens more injured. The prime minister added that the significant amount of police activities had been carried out over the last 24 house and there were 11 people in custody under the investigation.
"In the light of these developments, JTAC, the independent Joint Terrorism Analysis Centre, has this morning taken the decision to reduce the threat level from 'critical' to 'severe'," May said in a statement.
On Wednesday, May announced that the United Kingdom had upgraded its terror threat level to "critical," or the highest possible designation, meaning that an imminent terror attack in the country was anticipated.
According to MI5, the UK Security Service, the "severe" level means an attack is "highly likely."