David Cameron is "seriously concerned" about escalating tensions over Gibraltar, as the territory's chief minister accused its Spanish neighbours of behaving like North Korea.
The Prime Minister's spokesman said threats from Madrid to impose a €50 (£43.30) fee on every vehicle entering or leaving the British Mediterranean outpost through its border with Spain had not been raised directly with the UK Government.
The spokesman said: "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 they might target Gibraltar with further measures."
Gibraltar's leader Fabian Picardo accused Madrid of "sabre-rattling" following Spanish foreign minister Jose Garcia-Margallo's suggested levy on border crossings.
Mr Garcia-Margallo said the proceeds could be used to help Spanish fishermen who have lost out because of damage to fishing grounds allegedly caused by Gibraltarian authorities. Such a fee could impose punitive costs on Gibraltarians who regularly commute into Spain to work.
Mr Picardo said "hell will freeze over" before the authorities in Gibraltar remove an artificial reef that Madrid claims is harming Spanish fishermen, adding that any border costs would violate European Union freedom of movement rules.
Mr Picardo said: "What we have seen this weekend is sabre-rattling of the sort we haven't seen for some time.
"The things 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 claims sovereignty over the Rock, which is on the southernmost tip of the Iberian peninsula but has been a British Overseas Territory since 1713.
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