The threat posed by al-Qaida and the Taliban is exaggerated and the western-led counter-insurgency campaign in Afghanistan risks becoming a "long, drawn-out disaster", one of the world's leading security thinktanks warned today.

According to the International Institute for Strategic Studies (IISS), the west's counter-insurgency strategy has "ballooned" out of proportion to the original aim of preventing al-Qaida from mounting terrorist attacks there, and must be replaced by a less ambitious but more sensible policy of "containment and deterrence".

The critique of the US- and British-backed military policy is contained in the latest strategic survey from the IISS, a respected but usually uncontroversial body. IISS officers made clear today they have departed from their normal practice because of the serious threat to the west's security interests in pursuing the current Afghan strategy.

In an effort to ignite a fresh debate and bring about a new approach towards Afghanistan, they challenge claims, not least from David Cameron, that the presence of thousands of British troops in Afghanistan is necessary to prevent al-Qaida from returning and thus increasing the threat to the UK.

"It is not clear why it should be axiomatically obvious that an Afghanistan freed of an international combat presence in the south would be an automatic magnet for al-Qaida's concentrated reconstruction," the IISS director-general, John Chipman, said.

Al-Qaida is now "engaged in Pakistan in very small numbers", not remotely comparable to the situation in Afghanistan pre-September 2001, Nigel Inkster, an IISS director and former deputy chief of Britain's Secret Intelligence Service, said. No such threat is likely to come from al-Qaida elsewhere, including Yemen and Somalia, he added.