David Cameron 'seriously concerned' by Gibraltar events
Prime Minister David Cameron is "seriously concerned" about the escalation of tensions at the Spanish-Gibraltarian border.
Spain has said it is considering a range of proposals including a new 50 euro (£43) fee to cross the border with the British territory.
Mr Cameron said none of the measures had been raised with the UK government.
Spain's latest move follows increased vehicle searches at the border last weekend, causing major delays.
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.
Gibraltar's Chief Minister Fabian Picardo has accused Spain of "sabre-rattling".
He told BBC Radio 4's Today programme: "The things that Mr Garcia-Margallo has said are more reminiscent of the type of statement you'd hear from North Korea than from an EU partner.
"We've seen it before during Franco's time during the 1960s, but I think all of us hoped that those politics were never going to come back."
Gibraltar has linked Spain's hardline 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".
Mr Garcia-Margallo told the newspaper the proceeds from the border fee could be used to "help fishermen affected by the destruction of fishing grounds".
The newspaper article is the latest of several incidents in recent weeks that have escalated tensions.
In June, shots were reportedly fired by a Spanish Guardia Civil vessel at a jet skier in waters around Gibraltar.
Then at the end of last month residents and tourists had to endure three days of delays at the border as a result of increased vehicle searches by the Spanish authorities.
The Spanish government said it had a duty to prevent smuggling.
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.
Spanish daily newspaper ABC said there was "nothing strange about Spain's decision to exercise its rights and obligations regarding a border outside the Schengen zone", and it was "up to London to explain to the people of Gibraltar that they have to take this into account before they start throwing concrete blocks into waters that are not theirs".
An editorial in Diario de Almeria said Spain should consider removing the blocks "because not taking action would mean to once again surrender to the practices that Gibraltar and the UK have been using for three centuries".
The UK's shadow foreign office minister Kerry McCarthy said a border fee would be "unacceptable" and the Foreign Office must resist any efforts to impose one - or to use the border crossing "to score political points".
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, about Mr Garcia-Margallo's comments.
He asked Mr Barroso "for his personal intervention to stop such objectionable behaviour by an EU government bullying EU citizens".
"The European Commission must remind Madrid of its obligations under EU law," he added.