Expatriates living in Gibraltar and the surrounding area are becoming concerned about comments made by Spain's foreign minister about introducing a fee at the border.
Jose Manuel Garcia-Margallo said that the country was considering implementing a €50 (£43) charge to pass into the British territory in relation to a dispute about an artificial reef.
The minister said that proceeds from the fee would go towards those fishermen affected by the reef, which he says will destroy their fishing grounds.
Work on the reef started ten days ago, when concrete blocks were put in the sea.
The UK's foreign minister William Hague has expressed concern about the comments made by his counterpart.
As well as the proposed fee to be charged at Spain's border post, Mr Garcia-Margallo has also told the ABC newspaper that other measures may come into force.
These include launching an investigation into property in Spain owned by some 6,000 Gibraltarians and forcing online gaming companies operating out of Gibraltar to use Spanish servers, meaning they would have to comply with Madrid's taxation policies.
A spokeswoman for the UK Foreign Office said: "We are concerned by today's comments on Gibraltar, which we are looking into further."
Spain has officially lodged a complaint with the UK over the reef, but since the issue came to the fore a number of changes have been noted.
Last weekend saw delays at the border between Spain and Gibraltar as authorities increased the number of vehicle searches they carried out.
However, Spain denied that this was a reaction to the row and just an expression of its commitment to preventing smuggling.
Since then the delays have stopped, with the Foreign Office suggesting that Mr Hague had put a call in to Mr Marcia Margallo.
Gibraltar has been governed by Britain for 300 years, but its sovereignty has been challenged by Spain on numerous occasions.
The Foreign Office spokeswoman said: "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."
Meanwhile, expats remain concerned about the possible measures Spain may implement in relation to its border with Gibraltar.