The 39-page memo recovered from an al-Qaeda laptop computer in Pakistan three years ago read like an Idiot’s Guide to Bombmaking. Forget military explosives or fancy detonators, it lectured. Instead, the manual advised a shopping trip to a hardware store or pharmacy, where all the necessary ingredients for a terrorist attack are stocked on the shelves.

“Make use of that which is available at your disposal and...bend it to suit your needs, (improvise) rather than waste valuable time becoming despondent over that which is not within your reach,” counselled the author of the memo, Dhiren Barot, a British citizen who said he developed his keep-it-simple philosophy by “observing senior planners” at al-Qaeda training camps.

Barot, who was later captured near London and is serving a 30-year sentence, had envisioned an attack with multiple car bombs that would detonate liquid-gas cylinders encased in rusty nails—a strategy with striking similarities to an attempt by a suspected terrorist cell to blow up three vehicles in London and Glasgow last week.

Counter-terrorism officials have warned for years that Osama bin Laden and his lieutenants have tried to obtain weapons of mass destruction, such as a nuclear device or chemical or biological weapons. In response, US military and intelligence agencies have invested vast amounts of money to block their acquisition.

So far, however, al-Qaeda and its affiliates have relied almost solely on simple, homemade bombs crafted from everyday ingredients—such as nail-polish remover and fertiliser—when plotting attacks in Europe and the US.

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