With holiday season approaching, police in Zurich are warning parents that failure to present such authorisation could result in not being allowed to travel.
Police say many people are unaware that mothers or fathers travelling with kids but without their partners may be asked to provide evidence that they have permission to take those children out of the country.
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This is particularly true in cases where children have a different surname from the parent they are travelling with – an increasingly common occurrence.
The requirement to demonstrate that permission has been granted by the non-travelling parent is set out in the Schengen free movement treaty. The measure is designed to stop children being kidnapped and taken out of the country.
In reality, border police manning passport control have plenty of discretion when it comes to demanding to see this permission to travel, according to Zurich daily Tages Anzeiger.
Factors taken into account by border police include the destination of travel and the behaviour of the child or children. Officials may even speak to children if they have concerns, Blick newspaper reported.
But in the end border police have the final say and travellers who miss their flights because they fail to present the necessary paperwork have to foot the bill for later travel themselves, according to Blick.
Blick also cited a Swiss airline spokesperson as saying that it is not just border police who can demand to see a letter of consent: airline gate staff can also do so in the case of some travel to some countries.
There is no formal legal requirement to carry a document noting the authorisation of the other parent for a child to travel, which means there is no corresponding form to fill out, Tages Anzeiger reported.
However, to avoid problems, police recommended that parents carry a new letter of consent for each trip outlining the travel dates, the duration of the trip and the reason for travel. Providing a phone number for the non-travelling parent is also recommended.
Police also said it is a good idea to carry a photocopy of the passport of the parent not making the trip.