A spokesman for the Gibraltar port authority said the Trafalgar class vessel had sailed into port on Saturday morning but was unable to shed light on the purpose of the visit.

"She is alongside the berth. They (British naval and defence authorities) just tell when she is coming but not the purpose of the visit," the spokesman told AFP.

A British Royal Navy spokesman told AFP from London that the sub was making "a scheduled routine visit" and was on "operational standdown following participation in a NATO exercise."

He would not comment on the length of the intended visit for security reasons.

Spanish media quoted a spokesman from Ecologists in Action as describing the Trenchant as a "floating bomb" whose presence comprised a risk to people living along the coast.

Last month, the Spanish government said it was still waiting for Britain to express in writing a commitment London made four years ago to end visits by British nuclear submarines to Gibraltar, although Madrid recognised that the territory's interior waters were ceded by Spain to Britain in the 1713 Treaty of Utrecht.

Madrid expressed the hope that Britain, as an EU ally, would seek "to guarantee the security of the people who live in the area."

In May 2001, then Spanish foreign minister Josep Pique said his British counterpart Robin Cook had told him nuclear repairs to British submarines in Gibraltar would cease.

However there have been several such visits since, including last February by HMS Sceptre for maintenance, an arrival that drew a formal protest from Madrid and a complaint from Gibraltar, whose authorities said they were not told in advance of the visit.

Britain says Gibraltar provides an important support facility for maintenance and several other submarines have docked there in recent years.

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