Former Guantanamo Bay detainee Moazzam Begg said he was effectively given a "green light" to go to Syria by MI5 before being detained on terror charges on his return.
Mr Begg walked free from jail on Wednesday when he was formally acquitted in the light of "new material" just days before his trial was due to begin at the Old Bailey.
The 46-year-old launched an attack on British home and foreign policies as he left high-security Belmarsh prison after seven months in custody, accusing the Government of "demonising" Muslims.
Asked about reports that the case collapsed because of the revelation that the security services were aware of his activity, he told Channel 4 News he had met with MI5 to discuss the issue.
"My relationship to MI5 was at a level where they asked whether I'd be willing to speak with them about my previous visit to Syria," he told Channel 4 News.
"I told them I didn't think it was ethical if I speak to you, because I'm going there to investigate you and what you've been up to, and your relationship with the Assad regime," he said.
"Eventually we did meet with their lawyers and mine and at the end of that meeting it was very clear when I said to them that if I am prevented from going to Turkey and Syria, then I can assume safely it was from them that the prevention is happening. They say you will find no hindrance from our side.
"They certainly gave me the green light in the sense that they didn't say there was anything wrong with it and the crucial part is there were lawyers present on both sides."
Anybody travelling to Syria at that time "would be connected to fighters in one way or another", he said, when asked if the authorities had been aware of with whom he planned to stay.
His defence against the charges was that he was involved in training young men to defend civilians against war crimes by the Assad regime, not in fighting or training terrorists.
Mr Begg had denied seven counts - one of attending a terrorist training camp in Syria between 2012 and 2013, five of possessing terrorist documents and one of funding terrorism by making a generator available.
"I'm neither a fighter nor a trainer, but I helped and assisted those who were," he said.
He dismissed the idea that "one of the most scrutinised people in the world" could pose a security threat.
"I have just been imprisoned for seven months. I have been interrogated, I have been questioned, I have had the entire weight of the British establishment which includes all of its arms from the prison service to the police service to the intelligence service to the Home Office.
"Prior to that I was imprisoned in Bagram and Guantanamo. I was interrogated by the CIA, the FBI, by MI5, by military intelligence. I am probably one of the most scrutinised people in the world."
After the charges were dropped, a CPS spokesman said: "If we had been made aware of all of this information at the time of charging, we would not have charged."
The Home Office declined to comment on what it said was a security matter.