A Royal Navy warship and two support vessels arrived off the coast of Gibraltar today as the row over fishing rights with Spain escalates.
The British government insists the move was pre-planned, but did not rule out the use of political pressure if the matter with Spain is not resolved diplomatically.
ITV News' Europe Correspondent Emma Murphy reports
The president of the European Commission José Manuel Barroso has spoken on the phone with Spain's Prime Minister Mariano Rajoy about the tensions over Gibraltar:
– European Commission statement
They agreed that a Commission fact finding mission should as soon as possible examine in loco the border control/movement of people and goods questions.
President Barroso expressed his hope that Spain and the UK will address these matters in a way that is in line with their common membership in the EU.
Julie Girling, MEP for South West England and Gibraltar, told ITV News that locals had "no hope that things are going to get better soon."
Delays at the Spanish Gibraltar border were "entirely pre-meditated to cause the maximum disruption," she said.
She added that Spanish social media users had been using Twitter to insult her "personal experience, my private life, my health" but that she would not give up fighting for the Gibraltarians.
Downing Street declined to rule out the use of retaliatory political action if the situation was not resolved quickly.
It has been reported last that UK officials are examining the potential to disrupt Spain's lucrative tourist industry as well as blocking its policy initiatives at the EU.
Pressed repeatedly on the potential for such action, a Number 10 spokesman told reporters at a Westminster briefing:
Our preference here is to resolve this via political means and through dialogue with the Spanish government.
Asked if Mr Cameron was confident of securing a swift resolution, he said: "We will do what we need to do to bring this to a satisfactory conclusion."
He said the Prime Minister wanted the EU to send monitors to the border "urgently".
We are still actively pursuing legal action. If we do pursue it, the first step would be for the European Commission to investigate the issue. That is why the Prime Minister spoke to President Barroso.
The Commission needs to meet its responsibilities to uphold EU law.
Locals waving Union flags gathered on the quayside to watch the British warships come in.
Andrea Jones, 46, works for an online gaming company and has lived in Gibraltar for 12 years.
She said the frigate's arrival was "a two-fingered salute towards Spain".
"I think the Gibraltarians are tending to be a bit more passionate at this moment in time," she said.
Retired Royal Gibraltar Police officer Michael Sanchez, 53, said he would like to see the British vessels off Gibraltar more often.
"It is getting to be out of control, it is not a spat any more," he said.
"It's a normal deployment but we need bigger assets to show them."
A Spanish Guardia Civil patrol boat passed close to the military area of Gibraltar harbour not long after HMS Westminster arrived this morning.
The Spanish boat passed outside the harbour walls in Gibraltar Bay before speeding off when a police launch approached it.
Royal Navy Warship HMS Westminster has entered Gibraltarian waters ahead of a planned naval exercise.
The ship arrives a day after Spanish fishermen staged a protest against an artificial reef built in the area by the Gibraltar authorities.
A Royal Navy warship is expected to arrive in Gibraltar later after more than 40 Spanish boats were involved in a confrontation with the British authorities.
The arrival of HMS Westminster, a type 23 frigate, is not part of Britain's response to the growing row.
The vessel left Portsmouth naval base in Hampshire six days ago to join nine other vessels taking part in a pre-planned international training exercise called Cougar '13 in the Mediterranean and Gulf.
Cougar '13 is a long-planned deployment involving four Royal Navy warships, the lead commando group from 3 Commando Brigade Royal Marines and elements of naval air squadrons.
Spanish fishermen sailed into disputed waters off the coast of Gibraltar today to stage a stand-off with Royal Navy boats.
The row is over an artificial reef which Spain claims restricts their right to fish, but Gibraltar says they shouldn't be fishing there anyway.
ITV News reporter Helen Callaghan reports: