The Foreign Office is set to lodge a complaint with Spain after travellers attempting to cross into Gibraltar were forced to queue for three hours.
The queues were the longest delays this year, the Royal Gibraltar Police said, although some travellers claimed to have waited for up to five hours.
The Daily Telegraph reported there had been no delays leaving.
"The FCO will be protesting to the Ministry of Foreign Affairs about the unacceptable delays seen this morning at the Spanish border with Gibraltar," a police spokesperson told the newspaper.
The UK government is considering legal action against Spain over the border controls which have caused delays for both tourists and locals.
The border checks have been imposed in retaliation against Gibaltar's creation of an artificial reef in its waters, which Spain claims is designed to ruin fishing in the area.
A Spain foreign ministry spokesman previously said the checks would remain in place, insisting they were "legal, proportionate" and that the country was obliged to carry them out under the Schengen agreement.
Mariano Rajoy, the Spanish prime minister, said the country had not taken "any illegal steps".
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