The man, who calls himself Abu Mugheera al-Britani, is thought to have joined fighters allied to Al Qaeda in Syria, 'The Sunday Times' reported.
In an online article, he wrote: "I thank Allah for releasing me and providing me with the opportunity of carrying out jihad in his path again."
UK security services and Scotland Yard are said to be aware of the article written under his name and published in an Al Qaeda affiliated magazine called al-Risalah (Arabic for 'The Message').
The news comes within days of Islamic State (ISIS) claiming that Jamal al-Harith of Manchester blew himself up last Monday in an attack on coalition forces near Mosul in Iraq.
Al-Harith was one of 17 former Guantanamo inmates from the UK who shared a 20 million pound payout from the UK government in 2010 after they threatened to go to court claiming they had been mistreated at the US prison.
The newspaper has claimed that Al-Harith was part of a network of Guantanamo detainees who have sought to return to the terror battlefield in the Middle East.