London has raised concerns with Madrid after Gibraltar's government said vehicles leaving the British-held territory on Spain's southern tip had been made since to wait nearly six hours to get through new security inspections.
"Clearly we were concerned over the weekend by events on the border," the prime minister's spokesman told reporters.
"That is why the foreign secretary called his Spanish counterpart and my understanding is that the border crossings have now returned to normal. But clearly we will be monitoring the situation in coming days."
Britain's "priority" is to ensure "people's basic right to freedom of movement" at the border, the spokesman said.
The spokesman's comments came a day after Foreign Secretary William Hague telephoned his Spanish counterpart to express "serious concerns" and the Foreign Office formally protested to the Spanish ambassador in London.
Spain last week lodged a complaint with Britain over the building of an artificial reef in Gibraltar's waters, which Gibraltar said was necessary to stop incursions by Spanish fishing boats.
Gibraltar's chief minister Fabian Picardo said Spain had acted like a "bully" by inflicting the delays on travellers.
"It's quite inhumane in particular when you think about the fact that there are pregnant women, elderly people and some very young children in those vehicles," he told the BBC.
"Spain really has behaved quite unashamedly like a bully this weekend."
The government of Gibraltar said in a statement that an ambulance had been deployed to treat people with medical conditions who were stuck in the queue and that a Spanish man was taken to hospital with chest pains.
Britain has held Gibraltar since 1713 but Spain wants it returned and refuses to recognise British sovereignty over the waters off the land known as "the Rock".
Tiny Gibraltar, just 6.8 square kilometres (2.6 square miles) and home to about 30,000 people, overlooks the only entrance to the Mediterranean Sea from the Atlantic Ocean.