British Prime Minister David Cameron has now made it possible for parents to ask for their children's passports to be cancelled if they fear they could go overseas to join terror groups, the media reported on Monday.
The law already covers under-16s, but will be widened to 16 and 17-year-olds too, BBC reported.
Parents of children aged under 16 have earlier had the power to request the cancellation of passports - even where a child has taken or hidden the actual document - since July.
Families can contact a passport office where officials will investigate their concerns before a final decision is taken by the home secretary.
The government has already announced a number of other measures as part of its counter-extremism strategy, including bans on radical preachers posting material online and demands that internet service providers do more to remove extremist material and identify those responsible for it.
Home Secretary Theresa May said the passport measure had only been used by "a small number" of parents since July, but it was part of a package of new powers that would help tackle extremism.
She also acknowledged, however, that in some cases it was parents themselves who were choosing to take their children overseas to join terrorist groups.
Police estimate at least 700 people from Britain have travelled to support or fight for jihadist organisations in Syria and Iraq, such as the Islamic State terror group.
