* Britain says border checks are politically motivated
* Royal Navy warship sets sail for disputed territory
* Spain considers how to press claim to territory (Adds detail on legal options, edits)
By William James and Peter Griffiths
LONDON, Aug 12 (Reuters) - Britain warned Spain on Monday it might take legal action to try to force Madrid to abandon tighter controls at the border with the contested British overseas territory of Gibraltar in what it called an "unprecedented" step against a European ally.
The warning coincided with the departure of a British warship for Gibraltar, played down by the British and Spanish governments as part of a long planned, routine exercise but which underscored heightened tensions over the territory.
A spokesman for Prime Minister David Cameron said the Spanish border checks, imposed after Gibraltar created an artificial reef which Spain said blocked its fishing vessels, were "disproportionate" and "politically motivated".
Tensions over the rocky outpost at the mouth of the Mediterranean to which Spain lays claim have turned into one of the worst rows in years between the two European Union states.
"The prime minister is disappointed by the failure of the Spanish to remove the additional border checks this weekend and we are now considering what legal action is open to us," Cameron's spokesman said, arguing they breached EU law.
"This would be an unprecedented step," he added.
Opposition politicians in Spain have accused Prime Minister Mariano Rajoy of using the situation to distract Spaniards from the country's severe recession and a corruption scandal.
Spain said it would not back down over the border controls which it said were a legal and proportionate step to prevent money laundering and smuggling of tobacco and other products.
If Britain chooses to test whether those restrictions breach EU law on freedom of movement, the case is likely to end up in the European Court of Justice. Judgements there can take years, although special cases can be fast-tracked.
"There are some conceivable other international fora where you can discuss the issue but in terms of a proper litigation process the only one would be the European Court of Justice," said Mats Persson, director of the Open Europe think-tank.
The territory, which has a population of 30,000 and relies on tourism, the gambling industry and offshore banking, was ceded to Britain by Spain in the 1713 Treaty of Utrecht.
Spain's tougher checks at the 1.2 km (0.75 mile) border have caused long delays for thousands of tourists and local people. Madrid also aired the idea of imposing a border crossing fee and of banning planes using its airspace to reach Gibraltar.
A Spanish foreign ministry spokeswoman on Monday restated her country's position that it was considering through which international forum it could press its claim to Gibraltar.
HMS Westminster, a British Royal Navy warship set sail for the territory on Monday as part of an annual Mediterranean military exercise which both Spain and Britain say has been long planned and is unrelated to the dispute.
It evoked the 16th century naval rivalry between the two countries in which the English repelled an attempt by the Spanish Armada to try to invade England in 1588 and the Spanish defeated an English "Counter Armada" the following year.
A spokesman for the European Commission Jonathan Todd confirmed on Monday that a team of Commission officials would travel to Gibraltar in September to verify compliance with EU rules on frontier controls. (Additional reporting by Belinda Goldsmith, Fiona Ortiz in Madrid and Charlie Dunmore in Brussels; Editing by Andrew Osborn, Guy Faulconbridge and Philippa Fletcher)