New information has emerged from an intelligence operation about the possibility of an attack being planned in or near London, with subway stations and airports of particular concern to officials, a British security source told CNN Thursday…

A second British security source said a decision was made Wednesday night to raise the terrorism threat level from “substantial” to “severe” for the civil aviation industry and London transport.

The overall, national terrorism threat level remains at “severe.”

Increased security precautions have gone into effect at train and subway stations and Heathrow Airport, though an attack is not considered imminent, a second British security source said.