British jihadis heading to Syria are increasingly choosing to travel south through Italy and across the Mediterranean to north Africa in order to evade detection to reach the forces of Islamic State. The route is in the opposite direction of the main migration channel north for the tens of thousands of people attempting to cross the Mediterranean from Libya before moving up through Italy, often starting in Sicily.

Sources monitoring the movements of British jihadis say that to avoid security measures at UK airports Isis recruits are crossing the channel by ferry then heading to Italy from where they sail south to north Africa, frequently landing in Tunisia before crossing into Libya.

Libya is viewed as a favourable destination for British Isis recruits bound for Syria as it is increasingly coming under the control of Isis. The troubled north African state is now the third biggest stronghold for the group and follows a week in which it has consolidated its grip on its self-declared caliphate in Syria and Iraq.

Isis fighters in Libya recently took control of Sirte, the home town of Muammar Gaddafi, defeating forces loyal to the country’s unofficial administration in Tripoli.

“We are seeing more and more taking the route across the Mediterranean to Libya; it’s becoming established. When they get there they are looked after. You can even follow some of them on Facebook going through Italy,” said a source.

Earlier this month, an adviser to the Libyan government alleged that Isis fighters were being smuggled into Europe by traffickers in the Mediterranean, claiming that militants were hiding on boats filled with migrants. Officials in Italy and Egypt have previously warned that Isis fighters could reach Europe via migrant boats, although experts caution that it is very difficult to verify or assess such claims.

Closer to home, scrutiny remains on the number of secret messaging mobile phone apps that are reported to have been hijacked by terrorists who allegedly use them to persuade British children to join Isis. The apps, which allow anyone to send encrypted messages, can be downloaded free and can be opened only by a private key. Critics claim Isis recruiters are therefore able to share their plots in secret, but the apps’ owners argue that private communications are a “human right” for everyone.

The Home Office say that such developments confirm that the police and security services are “losing capabilities” to keep track of terrorists.

Home secretary Theresa May said: “Rapid changes in technology, and particularly the increasing importance of internet communications, means that communications data is not always available when it is needed.”

Concerns that such statements indicate a threat to freedom of speech follow details of a plan by May to introduce counter-extremism powers to vet British broadcasters’ programmes before they are transmitted.

Cabinet colleague, Sajid Javid, the culture secretary, wrote to David Cameron to say that he was unable to support May’s proposal to give Ofcom the new powers to take pre-emptive action against programmes that included “extremist content”.