The Counter-Terrorism and Border Security Act 2019 creates a criminal offence of entering or remaining in a “designated area” overseas.
Ministers unveiled the measure last year as part of efforts to boost authorities’ ability to tackle the threat from so-called “foreign fighters”.
The act allows Home Secretary Sajid Javid to designate an area, subject to parliamentary approval.
In order to use the power, he would need to be satisfied that it is necessary to restrict UK nationals and residents from travelling to or remaining in the area in order to protect the public from a risk of terrorism.
An individual found to have entered or remained in a designated area could face up to 10 years in prison if convicted.
Exemptions have been written into the legislation to protect those who have a legitimate reason for being in the area, such as journalism, aid work or attending the funeral of a relative.
The offence does not allow retrospective prosecutions of individuals who have gone overseas to take part in fighting, such as those who went to territory held by Islamic State, before returning to the UK.
But officials say it will assist if a similar outflow of fighters is seen in a future conflict.
The aim is to provide a deterrent effect and further avenue for bringing charges in cases where individuals are suspected of travelling for terrorist purposes.
More than 900 individuals “of national security concern” from the UK have travelled to engage with the conflict in Syria, the Home Office estimates.
Of these, approximately 20% have been killed whilst overseas and around 40% have returned to the UK.
Figures disclosed in the Commons last year suggested that only around one in 10 returnees has been prosecuted over “direct action” in Syria, although the Government says a significant proportion of those who have come back were assessed as no longer being of national security concern.
The power to designate areas is one of a string of new anti-terror measures that come into force on Friday.
The legislation also:
-makes it illegal to “recklessly” express support for a proscribed organisation;
-creates an offence of obtaining or viewing terrorist material over the internet;
-extends extra-territorial jurisdiction for some terror-related crimes;
-increases maximum sentences for a number of offences.
Mr Javid said: “These new laws give the police the powers they need to disrupt terrorist plots earlier and ensure that those who seek to do us harm face just punishment.
“As we saw in the deadly attacks in London and Manchester in 2017, the threat from terrorism continues to evolve and so must our response, which is why these vital new measures have been introduced.”
Britain was hit by five attacks in 2017, while police and security services have foiled 18 plots in the last two years.