A former senior MI6 officer has criticised the torture and abuse of terror suspects and says the US response to the threat posed by al-Qaida has been exaggerated and counterproductive.

Stinging criticism of the US is made in the Guardian by Nigel Inkster, assistant chief of MI6 until 2006.

In the article, which appeared originally in the International Institute for Strategic Studies journal Survival, Inkster and co-author Alexander Nicoll write: "It is surely not inspiring for radicalised people with the potential for violent action to see terrorists tried in ordinary criminal courts and sentenced to long prison terms." The authors, both senior IISS members, add: "But it surely is inspiring to them to see terrorists treated as a special class of prisoners to be held by the military, imprisoned without trial and tortured. This is the kind of treatment that makes jihadists believe that they can indeed be the fighters for a cause that they aspire to be."

Abandoning "ordinary standards of criminal justice" in terrorism cases can be counterproductive, they say. "On top of this, there is the argument that democratic values … are the [west's] best advertisement. Departures from such values have damaged America's … reputation."

Senior officers in MI6, and MI5 the domestic security service, at the time privately expressed serious concern about the Bush administration's "war on terror" and what many of them believed was an unlawful invasion of Iraq. Two months ago, Eliza Manningham-Buller, the former head of MI5, disclosed that Britain lodged a protest to the US about the treatment of detainees.

America's "frenzied, alarmist response" to the failed attempt by a Nigerian apparently to blow up a US airliner last Christmas Day "is hardly becoming for the most powerful nation on earth", Inkster and Nicoll said. "The lack of any sense of proportion simply serves to enhance the status of a terrorist group which is dispersed, quite small and cannot possibly threaten US sovereignty unless Americans connive in their own defeat."

The huge expense of Bush's "global war on terror", they said, had arguably caused more damage to the world economy than Osama bin Laden could have hoped for. "Nobody can forget the horror of 9/11, and it was inevitable that a government faced with such an outrage would respond in extreme fashion. Hindsight is easy, but if Bush had placed more emphasis on bringing those responsible to justice rather than on declaring an unwinnable 'war' against an undefined enemy, things might have turned out very differently."