British Foreign Secretary William Hague has told the people of Gibraltar that the United Kingdom stands 'shoulder to shoulder' with them amid an escalating row with Spain in a dispute over fishing rights.
Madrid has threatened to charge a toll to cross the border and prevent flights to Gibraltar using its airspace.
Mr Hague said he had spoken to Gibraltar's Chief Minister Fabian Picardo to reiterate Britain's commitment to the colony.
"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.
The BBC reports 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 on Tuesday that Spanish must fully respect EU law in regard to checks at the border with Gibraltar.
In an interview with the ABC newspaper published on Sunday, Foreign Minister Jose Manuel Garcia-Margallo said Spain was considering charging people to enter and exit Gibraltar.
He also hinted at the introduction of other measures, including tax investigations into property owned by Gibraltarians in neighbouring parts of Spain.
Gibraltar, an outcrop near the southern tip of the Iberian peninsula, has been ruled by Britain since 1713.