The sources said the sub would remain in port for routine maintenance after arriving in mid-afternoon and that the Spanish foreign ministry had been informed.

British nuclear-powered subs occasionally dock in Gibraltar, a British possession on the southern Spanish coast, but their visits have been a source of contention with Madrid.

Last year's visit by HMS Tireless led to a Spanish protest, with Madrid regarding its arrival as "unfriendly."

Spanish Foreign Minister Miguel Angel Moratinos said its presence showed a "lack of sensitivity" towards the Spanish people, and the Tireless left port two days early in view of the protest.

"I spoke with (British Foreign Secretary) Jack Straw earlier, and he guaranteed the submarine would stay for a week and said he would keep us informed of the situation," said Moratinos, who lauded the "transparency" London had shown over the visit by HMS Sceptre.

However, Moratinos underlined Spain's desire to be "vigilant and ensure there is no danger to Spaniards."

He added that in his view HMS Sceptre's visit "is not comparable to that of the Tireless," but added that Spain would still be asking Britain to halt such visits in future.

Spain was irked by a previous visit from the Tireless to Gibraltar in 2000 when it put into port for almost a year following a problem with its reactor's cooling system.

On that occasion people living in localities adjoining Gibraltar protested amid fears the sub could leak radioactive material.

A visit in October 2003 by HMS Turbulent also irked Madrid.

Britain argues that Gibraltar provides an important support facility for submarine maintenance.

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