A secret report written by the United Kingdom’s Defense Ministry and MI5, the domestic intelligence agency, is warning that terrorists connected to Al-Qaeda are planning “mass casualty” attacks in the country, the Telegraph is reporting.

The report also warned that “some thousands” of extremists, including some trained in terrorist camps, are operating in the United Kingdom. The majority are British citizens born in South Asia, specifically Pakistan. The director of MI5 warned in November 2007 that there were at least 2,000 supporters of terrorism in the United Kingdom and the number had increased by 400 over the previous year.

“The main extremist concentrations are in London, Birmingham, with significant extremist networks in the South East, notably Luton,” the report said.

The report further stated that improvised explosive devices, or IEDs, “are the most likely form of attack in the UK.”

The United Kingdom last suffered a major terrorist attack in 2005 when terrorists linked to Al-Qaeda suicide bombed three trains and one bus in London. Fifty-two British civilians were killed in that attack.