A Port Alert is a very useful mechanism in an emergency situation as you do not need to make a formal application to the Court or require an Order of the Court in order for there to be an All Port Alert in respect of a child.
An application simply needs to be made to the tipstaff who are the High Court Enforcement Officer for them to put an alert at all areas of entry and exit to the UK i.e. airports and ports.
You need to establish that there is a real and imminent risk that a child under 16 will be removed. If you establish this then the child can be placed on the abduction warning list for 28 days.
Applications for such Orders can also be made to the police.
Obviously if you have an Order of the Court such as a Wardship Order or a Location & Collection Order then there will be no doubt that the tipstaff or the police will implement a port alert.