Councillor Gerald Vernon-Jackson was leader of Portsmouth City Council when six men from the city left to fight with Islamic State last year.
But he said those returning should only get credit if they provide information.
Academic Joseph Carter, from Kings College London, said he was aware of UK fighters having a change of heart.
Two of the six men have since been killed while Mashudur Choudhury returned to the UK.
In May, he became the first person in the UK to be convicted of terrorist offences in connection with the conflict in Syria.
Foreign Secretary Philip Hammond recently said UK jihadists who travel to Iraq or Syria to fight could be tried for treason.
Mr Vernon-Jackson, who meets regularly with Portsmouth's Muslim community, said: "They will still go to jail. The law is the law and you can't escape that.
"It depends whether they've had a real change of heart... and are prepared to work actively with our security services."'Ignorant or confused'
Muslim journalist Dilly Hussain said many were influenced by television and social media images of Muslims suffering under the regime of President Assad.
He said: "Naturally, it will appeal to some disenfranchised, ignorant or confused young Muslims.
"They were seeing on television Syrians being barrel bombed, starved, killed, raped. They felt charity work wasn't enough and wanted to go out there and physically do something to help the oppressed."
Mr Carter, who is researching radicalisation, said: "We've seen a few fighters who've expressed interest in coming back because they say this is not what they bought into.
"They thought they were going to help the Syrian people and they were going to help overthrow Bashar al-Assad. And now you have people from the Islamic Front, Jahbat al-Nusra, and IS fighting each other."