The Sunday Telegraph can disclose that Michael Adebolajo was held by police close to the Somali border with a band of “radicalised” Muslim youths who wanted to join the notorious al-Shabaab group.

He was deported to Britain after he appeared in court in Mombasa in November 2010.

Two months previously the head of MI5 had warned that Britons were training in Somalia and it was “only a matter of time before we see terrorism on our streets inspired by those who are today fighting alongside al-Shabaab”. It also emerged that the other suspect in the soldier’s murder, Michael Adebowale, 22, was detained by police in London two months ago after shopkeepers complained about a group of Muslim activists.

The disclosures raise further questions about the monitoring by the security services of Adebowale and Adebolajo, 28, whom sources have said was known to MI5 but not assessed as a “threat to life”.