British extremists trapped in Syria face jail sentences of up to 10 years if they return to the UK, under a law that came into effect last month.
The legislation makes it an offence to enter or remain in "designated areas" outside Britain.
The British home secretary and foreign secretary will designate areas they believe UK citizens should avoid to protect them from the risk of terrorism.
After the area has been designated, there will be a grace period of a month before the offence takes effect, giving militants time to return to the UK to escape a harsher sentence.
So far, the only area covered by the legislation is Idlib, a province in north-western Syria that is partly controlled by extremists linked to Al Qaeda.
But the law is expected to extend far wider, with a new list of areas being discussed by MPs this year.
The Sunday Times reported that such areas would probably include large parts of Syria, as well as regions in Afghanistan, Libya and Somalia.
There are exemptions to the law, including whether a person has travelled to provide humanitarian aid, work as a journalist, attend court, work for the UN, work for the government of a country other than the UK, attend a funeral of a relative, or visit a terminally ill relative.
The offence would also not apply to a person who is in the service of the UK, such as British Army personnel.
Similar laws have been enacted in Australia and Denmark as a response to the conflict in Iraq and Syria.
The UK government said that more than 900 people who pose concerns to national security travelled from the UK to Syria.
Since 2014, large numbers of British nationals have travelled to Iraq and Syria to engage in terrorism.
In his evidence to the Public Bill Committee, assistant commissioner Neil Basu, the head of the UK's counterterror policing, said the law would help to tackle the threat posed by foreign fighters.
“In the scenario of a future conflict that was attracting foreign fighters from the UK, we see real operational value in a designated area offence," Mr Basu said.
"Such a power would have a deterrent effect to some of those who would seek to travel, and be a helpful disruptive and punitive tool on return."
