PORTSMOUTH (AFP) - The British warship HMS Westminster set sail on Tuesday on a training exercise that will include a stop at Gibraltar, the territory at the centre of a row between Britain and Spain.

The type 23 frigate left Portsmouth naval base on England's south coast to join nine other Royal Navy vessels heading for the Mediterranean and the Gulf.

Defence officials said the training exercise has long been planned and follows similar deployments in 2011 and 2012, but it comes at a time of heightened tensions over Gibraltar, a rocky outpost on the southern coast of Spain.

The British government on Monday said it was considering legal action over "totally disproportionate" border checks imposed by Spain on the British outpost, which have led to traffic tailbacks lasting several hours.

But Spain refused to stop the checks, which it countered were "legal and proportionate", and has said it was considering taking the row over the disputed territory to the United Nations and the International Court of Justice.

Relatives of sailors deployed on HMS Westminster waved off the warship from its home port as it began a seven-month deployment.

"Although it is pre-planned, I think they are sending a very strong message," said Stephen Kirk, who came to see off his granddaughter, 19-year-old sailor Samantha Kirk.

"It's all wrong what's going on but what is the answer if they cannot solve it diplomatically?"