Britain has said it was reviewing its military presence off Gibraltar following a lengthy stand-off between the Royal Navy and a Spanish ship, but denied it was resorting to "gunboat diplomacy".
The Foreign Office minister, Mark Simmonds, on Wednesday said he wanted to de-escalate the situation but would not put up with "bullying and intimidation" of the British territory at the mouth of the Mediterranean.
Britain summoned the Spanish ambassador on Tuesday to explain the most serious incursion for months in the waters off Gibraltar, which Spain has long claimed as its own.
A Spanish state oceanographic research ship, backed up by three military Spanish police boats, ignored orders to leave for 22 hours before departing on Tuesday, coming within 250 metres of Gibraltar's harbour.
Gibraltar's chief minister Fabian Picardo responded by urging Britain to deploy bigger military ships to deter similar incursions.
"We constantly review as to whether the Royal Navy's deployment around Gibraltar is accurate. We are reviewing that at this current time as well," Simmonds told parliament in an emergency debate on the incursion.
He said he wanted a "political solution" to the dispute, adding: "We do not believe that gunboat diplomacy and tit-for-tat escalation is in anyone's interest.
"Bullying and intimidation, wherever it occurs, in unacceptable and that is what appears to be happening in relation to Gibraltar."
Spain ceded Gibraltar to Britain in perpetuity in 1713 but has long argued that it should be returned to Spanish sovereignty, and the territory remains a source of diplomatic tension.
The Foreign Office has recorded a huge surge in the number of incursions into British territorial waters since the Spanish prime minister, Mariano Rajoy, took office in December 2011, and has also summoned the ambassador three times.