Britain may take legal action against Spain over tighter border controls at its contested overseas territory Gibraltar, Prime Minister David Cameron's spokesman said on Monday, escalating a row that has strained relations between the two countries.

Tensions over Gibraltar - the rocky outpost at the mouth of the Mediterranean to which Spain lays claim - flared up this month after Spain complained that an artificial reef being built by Gibraltar would block its fishing vessels.

Spain has imposed tougher checks at the frontier, causing long delays for thousands of tourists and local people. Madrid also floated the idea of a new border crossing fee and a ban on planes using its airspace to reach the territory.

Cameron's spokesman said Britain thinks the tighter border controls are "politically motivated and totally disproportionate" and should be stopped.

"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," the spokesman said.

A Royal Navy warship set sail for Gibraltar on Monday as the row between Britain and Spain over the disputed island territory escalated with London's outspoken mayor telling Madrid to take its "hands off our Rock". Gibraltar has been a source of tension since Spain ceded the territory to Britain in the Treaty of Utrecht 300 years ago. - Reuters