Powers to ban suspected jihadis from returning to the UK have been used for the first time, Home Secretary Amber Rudd has revealed.
Temporary exclusion orders make it unlawful for the subject to come back without engaging with UK authorities.
Ms Rudd refused to say how many times TEOs had been used, but confirmed "we have started to use them".
The Home Secretary also admitted the authorities do not know how many Britons had returned from fighting with Islamic State or other extremists in Syria.
Manchester suicide bomber Salman Abedi is reported to have travelled to Libya in 2011 during school holidays to join his father Ramadan in the fight against Muammar Gaddafi, and it has also been suggested Abedi "most likely" went to Syria but Turkish officials reportedly said they have no record of him travelling to the war-torn country.
Asked how many people were thought to have returned from the war-torn Middle East state, Ms Rudd told BBC1's Andrew Marr Show: "We don't know the exact number.
"What we do know, in engaging with the intelligence services and with the police and with the Border Force, we make sure that they have the tools to track them and to keep them out where we can."
The Home Secretary also said "good progress" was being made with online firms about finding ways to tackle terrorists using secure communication services such as Whatsapp.
"We are making good progress with the companies who have put in place end-to-end encryption," she said.
"Some of them are being more constructive than others and we will continue to build on that.
"The area that I am most concerned about is the internet companies who are continuing to publish the hate publications, the hate material that is contributing to radicalising people in this country."
She added: "What we are doing is challenging the people who are delivering end-to-end encryption to work with us so that we have a way of keeping people safe.
"Nobody wants terrorists to have a safe place to exchange information and to be able to plot their terrible atrocities.
"I believe we can get them to be more successful in working with us to find a way of getting some of that information."