Michael Howard, a former leader of the ruling Conservative Party, noted on Sunday that former PM Margaret Thatcher took military action after Argentine forces invaded the Falkland Islands 35 years ago and said current leader Theresa May would "show the same resolve" on Gibraltar.
Spanish Foreign Minister Alfonso Dastis said Madrid was taken aback by such comments.
"The Spanish government is a little surprised by the tone which the United Kingdom has adopted, a country known for being phlegmatic.
"On this subject, the traditional British phlegmatism is conspicuous by its absence," Dastis said.
May on Sunday insisted she would "never" allow Gibraltar to slip from British control to allay the fears of visiting Chief Minister of Gibraltar Fabian Picardo.
British foreign minister Boris Johnson added that "Gibraltar is not for sale."
He said on Monday that London's position was "very, very clear, which is that the sovereignty of Gibraltar is unchanged, and it's not going to change."
With a population of just over 32,000, Gibraltar has been a British overseas territory since 1713 but Spain has long laid claim to it.
Following Brexit, the European Union says Spain would have to agree to extend any trade deal between the bloc and Britain to Gibraltar, meaning Madrid could potentially block the latter's access to a trade accord.
Commercially, the tiny territory is heavily reliant on its small land border with Spain and last year's Brexit vote is a source of concern in Gibraltar, whose residents voted massively in favour of staying.
Dastis said Madrid is not out to make things more difficult at the frontier, telling the El Pais daily in an interview published Sunday there was "no intention to close the border."
Spanish dictator Francisco Franco closed the border with Gibraltar outright in 1969. Free travel between the two sides was only fully restored in 1985, ten years after his death.
After Britain voted to leave the EU, Madrid proposed shared sovereignty, arguing that would allow Gibraltar to stay in the bloc.
But Gibraltarians overwhelmingly voted down that idea in a 2002 referendum.