The UK is "something of a hub for the development of terrorists", the Government's former independent reviewer of terrorism legislation has said.
Lord Carlile QC said Islamic extremism "remains far and away the greatest, and the least predictable" terrorist threat to the UK and most terrorists were home-grown.
He highlighted several cases from 2010 where British citizens were accused of terrorist activity in the UK and abroad.
Writing the foreword to an overview of the UK's links to Islamism-inspired terrorism worldwide, Lord Carlile said: "We should not lose sight of the clear evidence provided by the report that a majority of terrorists in the UK are 'home-grown', and have not been trained abroad.
"Unfortunately the evidence reveals the UK to be something of a hub for the development of terrorists who export their activities to other countries: several examples are given from 2010."
He added: "Islamic extremism is not the only terrorist threat to the United Kingdom, but it remains far and away the greatest, and the least predictable."
According to the report, published by the Henry Jackson Society, seven out of 10 Islamism-related offences last year were perpetrated by British nationals, with almost half of those living in London.
It comes after a series of British citizens were involved in attacks. Suicide bomber Taimour Abdulwahab al-Abdaly, 28, who studied at the University of Luton, blew himself up and injured two people in an attack in Stockholm's shopping district in December last year.
A terrorist suspect killed in a drone attack in Pakistan in September last year was reportedly Abdul Jabbar, a British man tasked with leading an al Qaida group in the UK. And terror suspect Mahmoud Abu Rideh, who was previously under a control order in the UK, was killed in a missile strike in Afghanistan last year.
The report, Islamist Terrorist: The British Connections, also found most of those convicted of terror-related offences (54%) served less than three-and-a-half years in jail.