Gibraltar has vowed retaliations if Spain imposes full border checks.
Its first minister Fabian Picardo was speaking as talks on border checks were postponed until after the July 4 general election.
It means negotiations on a common travel area between the British territory and Spain will not be settled until later in the year, reported the Telegraph.
Speaking to the Gibraltar Chronicle, he said: “If these talks break down, sorrow might turn to anger quite quickly.
“People in Gibraltar need to understand that the alternative to a treaty is potentially the full application of the Schengen border code with all of the massive difficulties that that would entail, and for us to do it reciprocally.
“In other words, if the Schengen border code is applied to Gibraltar and to crossings from Gibraltar to Spain, the government of Gibraltar will apply the same rules to crossings from Spain to Gibraltar.
“We will do that more in sorrow than in anger.”
Since Brexit, usual border procedures were temporarily suspended to allow free movement, but Spanish officials have warned this cannot last forever.
Negotiators had hoped for a breakthrough on an agreement giving Gibraltar associate membership of the EU’s passport-free Schengen Zone last month, but this failed to materialise.
This would allow some 15,000 Spain-based workers to move freely to and from the Rock every day.
If not given associate Schengen status, Gibraltarians and Spaniards would need to have their passports stamped and would be limited to 90 days visa-free travel in each direction.
The last round of negotiations became stuck over the issue of Spanish police operating at the airport.
An EU commission spokesperson said: “The commission remains committed to reach an EU-UK agreement for the benefit of the whole region. The teams remain in touch.”
