Home Secretary Sajid Javid is to warn that he could use new powers to ban British nationals from Syria.
A new counter-terrorism act came into force in February which allows the home secretary to ban people from travelling to, or remaining in, designated areas.
Failure to comply can result in a maximum prison sentence of 10 years.
Mr Javid will use a speech later to warn British nationals who are currently in Syria without good reason that they “should be on notice”.
Speaking to senior security figures in central London, Mr Javid will set out for the first time how he expects to use the new Counter-Terrorism and Border Security Act.
He is expected to say: “I’ve asked my officials to work closely with the police and intelligence agencies to urgently review the case for exercising this power in relation to Syria, with a particular focus on Idlib and the north east.
“So anyone who is in these areas without a legitimate reason should be on notice.”
The north-western Syrian province of Idlib is the last remaining stronghold controlled by forces opposed to President Bashar al-Assad.
Mr Javid will say that the police and security services “have worked tirelessly” to identify people intending to join the Islamic State group overseas and prevent them from leaving the…