Britain will introduce new laws on Tuesday to try to stop airlines carrying passengers who may be travelling to join the Islamic State (ISIS), a junior minister said on Sunday, according to Reuters.

Security services estimate some 600 Britons have gone to Syria or Iraq to join militant groups, including the man known as “Jihadi John” who has appeared in several ISIS beheading videos. They are among  thousands of European Muslims who have reportedly flocked to the Middle East to join jihadist groups.

Under the proposed new laws, Home Secretary Theresa May would be able to prevent airlines from carrying passengers, including children, believed to be travelling to take part in “terrorism-related activity” on known routes, such as those into Syria, according to a Home Office statement.

“This important legislation will disrupt the ability of people to travel abroad to fight and then return,” James Brokenshire, a junior minister for security in May’s department, said in the statement quoted by Reuters.

“It will also enhance our ability to monitor and control the actions of those who pose a threat,” he added.

The rules would require airlines to seek permission to carry such passengers. An automatic system based on passenger lists provided by airlines would flag high-risk travelers and stop them boarding aircraft.

The new powers are part of Britain’s efforts to stop foreign fighters from entering Syria via commercial flights and come weeks after three London schoolgirls fled Britain to join up with ISIS through Turkey.

Turkish Airlines has previously said it was helping a government investigation into the case but that it was only responsible for checking visas, according to Reuters.

Prime Minister David Cameron has also urged internet firms to do more to tackle online extremism after it was revealed the three girls had used Twitter to contact other women involved with ISIS.

In November, two brothers became the first Britons to be jailed for terrorism training in Syria, after they had admitted conspiracy to attend a terrorism training camp in 2013.

A month later, a British mother of six who hoped one of her sons would become a jihadist was jailed for five years and three months after she used social media to encourage acts of terror in Syria.

The danger from radicalized Britons was illustrated in October, when a four-man ISIS terror cell was busted by police before apparently planning to behead one or several Britons on the streets of London.