The Gibraltar exit border has been engulfed by hours-long queues as Spanish authorities enforced detailed checks on British passport holders trying to enter Spain.
Britons holding a Blue Card - which accredits them as residents on the Rock - were asked to give their reason for crossing the border and entering Spain, according to Gibraltar's public television GBC.
Upon entering and leaving the Schengen zone, of which the UK was not part even before Brexit, Blue Card holders need to have their passport stamped.
At the discretion of the border officers and in accordance with European border control regulations, all non-EU citizens including Britons may also be asked to produce documents such as hotel bookings and tickets for return flights justifying the purpose and conditions of their stay.
Following Brexit, Britons can't remain in EU nations longer than 90 days over a 180-day period, unless they are in possession of a visa.
On November 20, waiting time at the border was up to two hours, according to the Spanish Huffington Post.
During the same weekend these long queues were recorded, King Charles's sister Princess Anne was carrying out an official visit to the Rock to attend the Gibraltar International Literature Festival.
These delays at the Spanish border came just days after 35 people, most of whom were British nationals, were refused entry into Spain from Gibraltar.
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Most of those barred from crossing the border on November 14, according to the Borders and Coastguard Agency (BCA), were holders of Blue Cards.
However, Spanish officials said to have prevented a from entering their nation also a Gibraltarian British national who failed to produce a Red Card ID.
Irregardless of Brexit, Red Card holders in Gibraltar are still allowed, pending the outcome of the stalling treaty talks between London and Madrid, to cross through the border with Spain without undergoing checks or having their passports stamped.
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Speaking about the incident, the BCA said: "Most were asked for the reason of their visit to Spain and for flights or hotel bookings.
"The BCA are not aware of any British national who presented their passport and a Gibraltar red ID card, to have been asked to provide reasons for travel.
"The Government understands that the existing status quo has not changed and that Gibraltar red ID card holders will not be required to stamp their passports when crossing the border to travel within Spain."