The reef of concrete blocks set off the coast at Gibraltar is apparently the cause of the current row and why Spanish customs are inspecting every car, making each driver open the boot to check for duty free cigarettes. It is also why Madrid is considering a 50 euro tax on every car.
Spain does not recognise Britain's right to territorial water around the rock and says that the reef is a violation of international law. They claim that it prevents Spanish fishermen from trawling in the waters. It turns out thought that the Gibraltar reef affects just one fishing boat, the Divina Providencia, which is moored in La Linea and was the only Spanish vessel to fish the area next to Gibraltar's airport.
In fact, seven years ago, Spain sank a reef of 90 concrete blocks just a few miles from Gibraltar at the request of local fishermen and there are 25 similar reefs of the coast of Andalucía. The reefs are designed to prevent trawling and thus allow fish stocks to recover.
The whole issue has now been taken up by the European Commission who say that Madrid must discuss the matter with the European Union. They plan to send a team of officials to the border for the purpose of carrying out spot checks. It will be September of October though before this team will arrive.