David Cameron has protested to his Spanish counterpart after shots were fired at a jet-skier off Gibraltar.
The Prime Minister confronted Mariano Rajoy during a Brussels summit and demanded an explanation for the "illegal incursion" into British territorial waters.
At a press conference, Mr Cameron said he wanted to make sure a full investigation took place and it was important that countries followed international law.
"I did raise this with the Spanish Prime Minister," he said. "I think this is a totally unacceptable episode and I made that clear.
"I think it is very important that people on Gibraltar know that we support them, we support their sovereignty."
He added: "We need to find out more about what happened but from what I have heard... it is not acceptable."
It is understood that Mr Cameron "made a beeline" for Mr Rajoy soon after arriving at the summit yesterday.
In the incident on Sunday, a Spanish police boat pursued Dale Villa while he was on his jet-ski near West Beach.
Witnesses reported seeing a large number of plastic batons being fired from the Guardia Civil vessel and splashing into the water. No-one was injured.