Imran Khawaja was arrested when he tried to return to Britain last year after attending a terrorist training camp in Syria and being photographed holding a man's severed head.
His guilty plea at the Old Bailey courthouse last month could not be reported until today when reporting restrictions were lifted.
He had admitted preparation of terrorist acts, attending a terrorism training camp, receiving weapons training and possessing a firearm for terrorism use.
Khawaja, from Southall, west London, had asked his taxi-driver cousin Tahir Bhatti, 44, to pick him up in Bulgaria and bring him back to Britain by car.
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Testimony indicated that Bhatti had convinced his cousin to leave Syria, in part by telling him that his parents were in poor health.
While they were driving, his death on the battlefield was falsely announced by associates in Syria.
Khawaja and Bhatti were arrested in June last year entering Britain at the port of Dover.
His cousin pleaded guilty today to assisting an offender and was granted bail. They will be sentenced next month.
A third man, 33-year-old Asim Ali, had also earlier pleaded guilty to providing Khawaja with money likely to be used for terrorism.