The terror group Islamic State (IS) had plotted thousands of attacks in Europe, plans discovered by coalition forces fighting militants in Syria and Iraq reveal.
Britain's top commander in the region said more than 10,000 documents and a large trove of digital data was seized after IS, also known as Daesh, were driven out of Manbij, northern Syria, in August.
"If we want to keep Britain safe, we need to deal with Daesh," said Major General Rupert Jones.
Speaking to reporters at the Al-Assad air base in Iraq, he said: "I am not going to go into the details but we know that external operations have been getting orchestrated to a very significant degree from within the caliphate, critically from within Raqqa and from within Manbij.
"They were key external operations hubs. There is a huge amount of intelligence, documentation, electronic material that has been exploited there that points very directly against all sorts of nations around the world."
British security agencies are now working to analyse the plots and will be expecting fresh intelligence if the coalition retakes Mosul in Iraq in its current offensive.
"I am absolutely certain that an extraordinary amount of intelligence will come out of (there)," added the commander.
"It will be a labyrinth of intelligence and we need to get that into the hands of the intelligence agencies."