James Matthews, from east London, was due to face trial in November after being charged with receiving training in Iraq and Syria on or before February 15 2016 "for purposes connected to the commission of preparation of terrorism".

The 43-year-old was believed to be the first person to be prosecuted for terrorism in Britain for assisting a group already helped by the British government.

But state prosecutor Tom Little said at a hearing at London's Old Bailey on Tuesday that there was no longer a realistic prospect of conviction on "evidential grounds".

Hundreds of foreign fighters from countries including Britain, Canada, France, Germany and the United States have fought alongside the Kurdish People's Protection Units (YPG) in northern Syria.

The group is a key component of the US-backed Syrian Democratic Forces alliance which waged a successful campaign against Islamic State jihadists.

Joel Bennathan, who was defending Matthews, said his client was "happy" charges had been dropped.

"We have always said the decision to prosecute Mr Matthews for fighting with the YPG against Isis was extraordinary and totally unjustified," he said.

"Mr Matthews is happy this has now come to an end.

"Mr Matthews was always open about what he had done and it is baffling that the CPS (Crown Prosecution Service) took two years to decide to prosecute him, then seven months later they have suddenly realised there is not enough evidence to do so."

The ex-soldier said he joined the YPG after seeing a photograph of an IS fighter holding the decapitated head of a woman.

"It seemed like one of the most single evil things I've ever seen and it affected me quite a lot," said Matthews, who appeared on a Channel 4 documentary called "The Brits battling ISIS".