Britain, France, Russia and the United States provided most of the weapons being used by ISIS to carry out appalling war crimes, a new report claims.
As the UK joins its allies in carrying out airstrikes in Syria, the Government has come in for renewed accusations of having sown the seeds of conflict itself.
A new study by Amnesty International claims much of the weaponry used by Islamic State (IS) fighters was supplied by the same Western powers now trying to bomb it out of existence.
Huge supplies of guns, rifles, mortars and missiles have been flowing into Iraq for years as the state struggled to retain control after British and American forces withdrew.
Read more: Anonymous hackers claim security services 'lack knowledge or manpower' to take down ISIS
Oliver Sprague, Amnesty UK’s Arms Programme Director, said: "Decades of free-flowing arms into Iraq meant that when IS took control of these areas, they were like children in a sweetshop.
"The fact that countries including the UK have ended up inadvertently arming IS, should give us pause over current weapons deals.
"The UK was one of the key supporters of a global Arms Trade Treaty, intended to prevent the proliferation of arms and their use in horrific abuses like IS are committing."
"Risks need to be far more carefully calculated, and we shouldn’t wait for this worst-case-scenario to happen before acting to prevent sales of arms which could fuel atrocities."
Although Soviet Kalshnikovs are the most commonly used, the oldest piece of kit in the IS arsenal is believed to be a British World War One rifle.
Amnesty says IS fighters acquired a windfall of internationally manufactured arms after taking control of Mosul, Iraq’s second largest city, in June, 2014.
They included US-manufactured weapons and military vehicles which they then used to take control of other parts of the country.
Among the advanced weaponry IS has seized are man-portable air defence systems, guided anti-tank missiles and armoured fighting vehicles, as well as assault rifles like the Russian AK series and the US M16 and Bushmaster.
Most of the conventional weapons being used by IS fighters date from the 1970s to the 1990s, including pistols, handguns and other small arms, machine guns, anti-tank weapons, mortars and artillery.
Arms and ammunition used by IS have been traced to at least 25 different countries, the report claims.
Amnesty says IS fighters and other armed groups have also resorted to forging their own improvised weaponry in crude workshops.
Examples include mortars and rockets, improvised hand grenades, explosive devices (IEDs) including car bombs and booby-traps, and even repurposed cluster munitions, an internationally banned weapon.
As a result of its findings, Amnesty is calling on all states to adopt a complete embargo on Syrian government forces, as well as armed opposition groups.
It comes as Britain began airstrikes on Syria last week after David Cameron won a vote in the House of Commons.
British RAF jets have joined France and the US in carrying out bombing raids on ISIS targets after the move was given the green light by Parliament.
Chancellor George Osborn claimed the vote proved Britain has "got its mojo back" and stands alongside the United States in the fight to "reassert Western values".
Speaking in the US, the Chancellor said it was a "source of real pride" for him that MPs had overwhelmingly backed air strikes in Syria against Islamic State.
Mr Osborne said Britain, in partnership with the US, was taking the fight to IS by cutting off financing and tackling extremist ideology.
He told the Council on Foreign Relations think tank: "Britain has got its mojo back and we are going to be with you as we reassert Western values, confident that our best days lie ahead."
