Spain 'must follow EU law' on Gibraltar border checks
Spanish checks at the border with Gibraltar must "fully respect EU law and remain proportionate", the European Commission has said.
Tensions have escalated after increased vehicle searches caused major delays for tourists and locals.
Spain has also said it is considering a new 50 euro (£43) fee to cross the border with the British territory.
The foreign secretary says he has told Gibraltar the UK stands "shoulder to shoulder" with it amid the tensions.
William Hague said he had spoken to Gibraltar's Chief Minister Fabian Picardo to reiterate the UK's commitment to the people of Gibraltar at "time of increasing Spanish pressure and rhetoric".
"We agreed that it was important to respond to actions, not rhetoric, and I confirmed that we would continue to raise our concerns with Spain," he said in a statement.
Earlier, Prime Minister David Cameron said he was "seriously concerned" about the escalating tensions.
Spain disputes UK sovereignty over Gibraltar, a limestone outcrop near the southern tip of the Iberian peninsula, which has been ruled by Britain since 1713.
Vehicles were delayed for up to seven hours at the end of July because of stricter checks by Spanish border guards. Spain has argued it has a duty to prevent smuggling.
The European Commission said Gibraltar was not part of the passport-free Schengen zone and therefore Spain was "obliged to carry out checks on persons and on goods", but must abide by EU law.
"The commission urges both Spain and Gibraltar to allow for smooth and efficient border crossings while taking into account these obligations," it said in a statement.
The commission said it had received assurances at the beginning of 2013 that border checks would be "proportionate", but said it had suggested "a technical meeting with the Spanish authorities in order to address the issues raised by the border checks, including the alleged excessive waiting times".
In an interview with the Spanish ABC newspaper, published on Sunday, Foreign Minister Jose Manuel Garcia-Margallo said Spain was considering charging people to enter and exit Gibraltar through its border post.
Mr Garcia-Margallo also hinted at the introduction of other measures, including tax investigations into property owned by Gibraltarians in neighbouring parts of Spain, and the closing of Spanish airspace to flights heading to Gibraltar.
Mr Cameron said none of the measures had been raised with the UK government, while Mr Picardo has accused Spain of "sabre-rattling".
Gibraltar has linked Spain's tough stance to the creation of an artificial reef in waters off Gibraltar.
Last month, Gibraltar dropped 70 concrete blocks into the bay to create the reef, but Spanish fishermen, who trawl the area for shellfish, say the move has prevented them working.
Spanish authorities said the blocks had been laid "without the necessary authorisation".
BBC political correspondent Alan Soady said Mr Cameron's government believed the reef dispute was a matter for Gibraltar and so felt it would be inappropriate to be directly involved in decisions on the fishing issue.
Instead, he said a Foreign Office source had told him it was involved "from a foreign policy perspective and to provide any support or advice the government of Gibraltar asks for".
Nevertheless, he said, the UK government wanted to be seen to be responding to the border checks issue, hence the serious language being used by the prime minister.
The European Commission said it did not have enough information to comment on the proposed border fee, but the UK's shadow foreign office minister Kerry McCarthy said any such move would be "unacceptable".
Earlier, Sir Graham Watson, a Lib Dem MEP for the South West of England and Gibraltar, said he had written to Jose Manuel Barroso, president of the EU Commission, asking "for his personal intervention to stop such objectionable behaviour by an EU government bullying EU citizens".