Both Islamic State and al-Qaida are calling for ‘frustrated travellers’ to stay at home and attack in the west instead

Britain has become more vulnerable to terrorist attacks because “frustrated travellers” are finding it more difficult to get to Syria and are being urged by different extremist groups to commit atrocities in the UK.

The return of some British fighters from Syria has added to a complex picture, which means the UK’s counter-terrorism agencies are feeling heat from all sides, intelligence sources said.

Let them leave.