A Spanish Navy fisheries patrol vessel was seen inspecting a Spanish fishing boat just off Gibraltar last week, in a move that could further raise tensions over the issue of territorial waters.

The incident occurred just a day after the Gibraltar Government confirmed that it had filed a legal challenge against the European Commission following a diplomatic row about the waters around the Rock.

On Thursday morning last week, the Government said it was taking the Commission to court over its decision to allow Spain to designate Gibraltar waters as a protected site under EU nature legislation.

At around 10am the following day, the Spanish patrol boat 'Tarifa' was spotted in Gibraltar waters off the east side of the Rock.

The vessel held position and lowered a small boat with

navy personnel on board to inspect the fishing gear on a Spanish fishing boat in the area.

Although a patrol boat from the Gibraltar Squadron rushed to the scene, the Spanish vessels left the area without further incident, according to sources in Gibraltar and Spain.

Spanish vessels - as with any ships - have the right of free passage through Gibraltar waters, but this sort of action by the Spanish Navy is highly unusual.

Until yesterday, officials on either side of the border had kept a tight lid on the incident.

And although there has been no official comment to date, opinion on the events differed between the sources contacted by the Chronicle.

One source described it as "quite a serious incursion", while another played it down and said "it wasn't much".

What some observers here fear is that the presence of the Spanish patrol boat was a reaction to the legal challenge over the designation of Gibraltar waters.

The row over the waters stems from a decision by Brussels to approve a Spanish request to list most of Gibraltar's territorial sea - and an area of international waters too - as one of Spain's protected sites under EU law.

In doing so, the Commission ignored the fact that Britain has jurisdiction over these waters and had already designated part of them as a European protected site in 2006.

The Spanish designation - in theory at least - sets out legal obligations and responsibilities on Spain over Gibraltar waters in relation to nature protection.

That includes responsibility to undertake assessments and monitor these areas for wildlife threats, and other activities with environmental implications.

The Spanish Navy describes the Tarifa's mission as "inspecting, monitoring and supporting the fishing fleet and protecting national fisheries interests."

The Chronicle was unable to reach anyone at the Foreign & Commonwealth Office or the Gibraltar Government for comment last night.

A spokesman for the Ministry of Defence said the Gibraltar Squadron had monitored the "Spanish activity" on the morning in questions but would add no further detail.

A spokesman for the Royal Gibraltar Police was aware of the incident but could not comment further because the RGP marine section had not attended the scene.