British volunteers fighting against so-called Islamic State in Syria pose a domestic security threat to the UK, a foreign affairs think tank has said.
Since 2014, hundreds of people from Western countries, including the UK, have joined Kurdish rebel group YPG.
The Henry Jackson Society said YPG was a front for the banned PKK group and warned that those who joined it could be drawn into terror activity at home.
It said the government urgently needed to stop Britons joining the group.
Security minister Ben Wallace said: "We deter all UK citizens from travelling overseas to fight with any form of irregular forces.
"People who do so should be aware that they potentially open themselves up to a range prosecutions linked to such behaviour."
Firearms and explosives
The Centre for the Response to Radicalisation and Terrorism, at the Henry Jackson Society, said the YPG was a subsidiary of the PKK, a "violent terrorist organisation" banned by the UK and the US.
The foreign affairs think tank said there was a danger those who fought for the group could participate in its "criminal-terror activities" or carry out "lone actor atrocities" when they returned home.
The training they are given in firearms and explosives also posed a risk, the report said.
Four British men have been killed fighting with the Kurds against IS and 29 Western volunteers have died in total.
Kurdish fighters on the ground in Syria have driven IS out of large areas of territory.
Kyle Orton, a fellow at the centre, said: "Far from battling terrorism they're in effect aiding one proscribed terrorist organisation overcome another.
"The government needs to act now to prevent Britons falling into this trap.
"If they're already there and want to come back, comprehensive tests need to be put in place to prevent potentially dangerous individuals within our communities."
'Freedom and democracy'
The father of a British man who died fighting with the YPG against IS in March 2015 told the BBC he was "deeply hurt" by the think tank's report.
Chris Scurfield, the father of Erik Scurfield, the first British man to be killed fighting against IS with the Kurds, said the report had upset families of British volunteer fighters.
He said: "We are speaking with other bereaved parents whose children also lost their lives in Syria, fighting alongside coalition forces to defeat ISIS and save the lives of civilians."
In a statement the Kurdistan Solidarity Campaign said: "British fighters who join the Kurdish units of YPG are not terrorists.
"They are fighting for the same values as British people hold dear, against the ISIS threat and for freedom and democracy. The YPG are in coalition with the UK and US forces.
"The Kurds do not encourage Brits to travel to Syria to fight ISIS, however many do. They are seen as heroes and to label them as this ill-informed report does is wrong."
The issue of foreign fighters has become a central security and political question for the UK since the rise of so-called Islamic State.
British volunteers travelling to fight against IS with the Kurds have been likened to Britons in the 1930s who went to fight against fascism in the Spanish civil war.
Authorities continually warn against going to fight for any group, but no returning fighters have yet been prosecuted specifically for joining Kurdish militia.
The YPG and PKK are seen as two different organisations by the British government.
The YPG is recognised as an important ally for the West and is currently fighting a difficult ground war to push IS out of the key city of Raqqa.
But whether to charge returning fighters is still being reviewed by the police and Crown Prosecution Service on a case-by-case basis.
Some say they go to fight for humanitarian reasons, while others are attracted by the Kurdish left-wing ideology.