The Foreign Office has said it is concerned that Spain may be taking a harder line over Gibraltar after comments made by Madrid’s Foreign Minister.

The worries arose after Jose Garcia-Margallo said “the party is over” for Gibraltar, adding that the previous Socialist government had been too soft on the British outpost which has long been a source of tension between London, the territory’s residents, and the Spanish.

The minister suggested that a €50 (£43.40) fee could be imposed on every vehicle entering or leaving Gibraltar.

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The proceeds would be used to help Spanish fishermen who have lost out because of damage to fishing grounds allegedly caused by Gibraltarian authorities.

Such a high charge would effectively rule out Gibraltarians commuting to Spain for work.

Mr Garcia-Margallo added that Spain could refuse to let flights to and from Gibraltar enter its airspace, and that it could change the law so that online gaming companies operating from the British overseas territory have to use Spanish servers and come under the jurisdiction of Madrid’s taxation regime.

The foreign minister added that Spanish tax authorities could launch an investigation into property owned by around 6,000 Gibraltarians in neighbouring parts of Spain, as part of its EU obligations to control “fiscal irregularities”.

The Foreign Office said: “We are concerned by today’s comments on Gibraltar, which we are looking into further.

“As we have said, we will not compromise on our sovereignty over Gibraltar, nor our commitment to its people. We continue to use all necessary measures to safeguard British sovereignty.”

Spain has already ramped up border checks, leaving some drivers waiting for up to seven hours in searing heat.

Additional reporting by agencies