London, July 15 (PTI) Up to 4,000 Islamic extremists from the UK have attended terrorist training camps in Afghanistan before returning to the country, a media report said today quoting security chiefs.

The alarming figure raises fresh questions about UK border controls and the capacity of the intelligence and security services to keep the country safe, The Sunday Telegraph said.

Quoting a senior security source, the report said "There are 3,000 to 4,000 people who went from the UK to Afghanistan and came back. The important question is, where are they now?"

The figure is more than double the estimate of 1,600 that Mi5, the British intelligence service, gave last autumn for the number of individuals actively involved in plotting terrorist attacks in Britain.

Afghanistan was the centre for al Qaeda terrorist training between 1996, when the Taliban came to power, and the end of 2001, when America and Britain invaded. Since then the focus has shifted to areas of Pakistan along the Afghan border.

More than 4,00,000 journeys are made each year between Britain and Pakistan, the vast majority of them legitimate. It is not known how many travellers continue their journey overland into Afghanistan.

Terrorists who are believed to have trained in Afghanistan include Richard Reid, from Bromley, Kent, and Saajid Badat, from Gloucester, who are both in jail for plotting to blow up an aircraft with shoe bombs.

Andrew Rowe, a Londoner of Jamaican origin serving 15 years for terrorist offences, was trained at one such camp in Afghanistan. Dhiren Barot, an NRI brought up in London and now serving 30 years for plotting a "dirty bomb" attack in Britain, was trained in either Afghanistan or Pakistan.

Others believed to have trained in Pakistan include Mohammad Sidique Khan, the leader of the July 7, 2005 bombing; Muktar Said Ibrahim, the leader of the failed July 21 bomb plot and most members of the "Operation Crevice" plot to blow up nightclubs or shopping centres with fertilizer bombs.