An alert issued by security services warned of plots against transport hubs by al-Qaida's "international operations wing" designed to cause "a large number of casualties," said the Daily Telegraph, which said it had seen the "restricted" document.
The newspaper said the document details plans for "one or more attacks against Europe, including the U.K., aimed at "high-profile Western targets."
The document said terrorists could use vehicle-borne or hand-delivered bombs and firearms.
The Department for Transport sent the security memo to airlines, airports and cargo carriers, and described the threat as "credible," the Daily Telegraph said.
The memo originated at the Joint Terrorism Analysis Center, which is part of MI5, which is the U.K.'s counter-intelligence and security agency.
"No announcement about the aviation or London transport sector threat level changes will be made to the media," the document said.
The threat level in Britain was raised from "substantial" to "severe" for transport hubs, meaning an attack is "highly likely," The Guardian said.
The report said fears are growing that Islamic extremists would attack crowds, killing indiscriminately.