Britain's anti-terrorism agency was on high alert Sunday after India shared its intelligence information about a suspected terror plot to hijack an Indian plane and crash it into a British city.
British intelligence agency MI5 was told by the Indian authorities early last week about a suspected plot by militants linked to Al Qaeda in Pakistan to hijack an Air India or Indian Airlines flight from Mumbai or Delhi.
The warning, which came after the arrest of a suspected terrorist, was contained in a detailed "threat assessment" by India's Intelligence Bureau, The Sunday Times reported.
It did not state that Britain was a specific target. But security sources said it had raised fears that a British city might be attacked.
Home Secretary Alan Johnson revealed Friday that the threat level to Britain was being raised from "substantial" to "severe". That is the second-highest level and means that an attack is "highly likely".
The move comes after the Christmas Day attempted suicide attack on an airliner over Detroit in the US.
A senior Whitehall official said that the Detroit attack proved that Al Qaeda had both the "capability and the intent" to attack western aircraft.
He said that the Joint Terrorism Analysis Centre (JTAC), a unit of MI5, had based its assessment on a broad range of factors including "an accumulation" of new material.
However, while insisting that there was no specific intelligence suggesting an attack, senior counter-terrorism officials said that the Indian warning was "a factor" in the move, the paper said.
Officials also say that the CIA warned two weeks ago about a possible plot by Al Qaeda in the Arabian Peninsula, the group behind the aborted plane blow-up attempt, to attack a second aircraft using terrorists trained at its camps in Yemen.
The threat to hijack an Indian aircraft was uncovered during the interrogation of Amjad Khwaja, a leader of Harkat-ul-Jihad-al-Islami, a militant group involved in terrorist attacks in India.
British officials are also concerned after Prime Minister Gordon Brown's recent statement over aviation security.
Following a meeting with spy chiefs and just after the Indian warning was received, he told MPs: "We know that a number of terrorist cells are actively trying to attack Britain and other countries".