The EU is to send a "fact finding mission" to the Gibraltar-Spain border to investigate checks imposed on the Spanish side amid continuing tension between the UK and Spain over the issue. The warship HMS Westminster has arrived in the area.
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:
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".
It may be a pre-planned training exercise but what timing: UK warships sail through disputed water to Her Majesty's naval base on Gibraltar.
Spain could damage to the economy in Gibraltar, but in doing so it would harm its own, so a further escalation of threats is unlikely.
The PM has said he is seriously concerned by Spain's plans to introduce a levy at the border with Gibraltar.