Cameron is 'seriously concerned' about mounting Spanish aggression over Gibraltar Fabian Picardo hits back at Spanish aggression over borders and air space Spanish threat to charge thousands of workers £85 a day to cross border Gibraltar authorities is building a reef to bar Spanish fishing boats David Cameron 'seriously concerned' but questions mount over response David Cameron is 'seriously concerned' about mounting Spanish aggression towards Gibraltar, after threats to impose border charges and ban flights from airspace. The Prime Minister is demanding an explanation after Spanish foreign minister Jose Garcia-Margallo threatened to charge thousands of workers £43 to cross the border. The chief minister of Gibralta, Fabian Picardo, accused Spain of acting like North Korea and "sabre-rattling" over the country's new hard-line stance over the Rock. Alarm: David Cameron is demanding an explanation over threats from Spain to impose border charges on Gibraltar Downing Street said proposals for fees or airspace restrictions have not been raised with the UK by the authorities in Madrid. The Prime Minister's spokesman said: 'Clearly, we remain seriously concerned by the events at the Spain/Gibraltar border. Specifically on this issue of border fees, the Spanish have not raised the prospect of introducing border fees with us. We are seeking an explanation from them regarding reports that they might target Gibraltar with further measures. The Government is in 'close contact' with the Spanish about the issue, but the spokesman refused to give further details about what the next steps might be. Row: Gibraltar minister Fabian Picardo (left) hit out at the aggressive stance of Spain's foreign minister Jose Garcia-Margallo who threatened to charge thousands of workers £85 a day to cross the border The row from Madrid follows a month of deepening tensions which began when Gibraltar authorities began building a reef to bar Spanish fishing boats. Mr Garcia-Margallo claimed the £43 entry fee could be used to help Spanish fishermen who have lost out because of damage to fishing grounds allegedly caused by Gibraltarian authorities. Planes heading for Gibralktar could also be barred from Spanish airspace. But today Mr Picardo said "hell will freeze over" before the authorities in Gibraltar remove the artificial reef. He added that any border costs would violate European Union freedom of movement rules. Mr Picardo told the BBC Radio 4"s Today programme: "What we have seen this weekend is sabre-rattling of the sort that we haven't seen for some time. 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. Spain is also considering closing its airspace to flights heading to the Rock. Mr Picardo claimed such a move would be dangerous and said it was the "politics of madness." Tensions: The British Foreign Office fears the proposed Spanish border charges would penalise islanders who regularly commute to Spain for work Queues: A woman gets out of a car as she waits in line to enter to the British territory of Gibraltar at its border with Spain He added: "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 and that the much more enlightened politics of Mr Moratinos (Miguel Angel Moratinos), who was the previous but one foreign minister of Spain, would prevail, which talked about people working together and creating economic benefits for the citizens on both sides of the frontier rather than the belligerence we are seeing now." Spain's main opposition party, the Spanish Socialist Workers Party (PSOE), criticised what it said was 'an escalation of bravado' between the governments of Spain and Gibraltar. Juan Moscoso del Prado, PSOE spokesman for EU affairs, said that the situation was leading to a 'dead end' and accused Mr Garcia-Margallo of 'dynamiting' dialogue on issues which affect workers, fishermen and traders. In an interview with Canal Sur radio station reported by the Europa Press news agency he said: 'You cannot threaten 50 euro fees when there are more than 10,000 people who go there every day to work. Workers should not be taken hostage by this conflict. The Foreign Office voiced concerns over Mr Garcia-Margallo's comments and said Britain would not compromise its sovereignty over Gibraltar. But the UK Government has made clear that it will not negotiate over sovereignty as long as Gibraltar's people want to remain British.