LONDON—Britain’s spy agencies have a new message for terrorists: Make cupcakes, not war.

Intelligence agents managed to hack into the extremist Inspire magazine, replacing its bombmaking instructions with a recipe for cupcakes.

It’s the first time the agents sabotaged the English-language magazine linked to US-born Yemeni cleric Anwar al-Awlaki, an extremist accused in several recent terror plots.

The quarterly online magazine, which is sent to web sites and e-mail addresses as a pdf file, had offered an original page titled “Make a Bomb in the Kitchen of Your Mom” in one of its editions last year. The magazine’s pages were corrupted, however, and the instructions replaced with the cupcake recipe.

“We’re increasingly using cybertools as part of our work,” a British government official who spoke on condition of anonymity to discuss intelligence matters said on Friday, confirming that the Inspire magazine had been successfully attacked. The hackers were reportedly working for Britain’s eavesdropping agency, GCHQ, which has boosted its resources in the past several years.

Despite sharp cuts to defense and intelligence agency budgets, Britain last year authorized an extra £500 million ($815 million) in funding on cyber-related projects. In London talks last month, Prime Minister David Cameron and US President Barack Obama also pledged to work more closely on cyber research.