Spain-Gibraltar row: A quick guide to the history
5 August 2013 Last updated at 23:58 BST
The Prime Minister says he has "serious concerns" over Spanish proposals to charge a fee for crossing the border into Gibraltar.
Spain has said it is considering a range of proposals including a new 50 euro (£43) fee to cross the border into the British territory.
But what's the background to the escalating tensions?
Tom Burridge explains.