Foreign Secretary William Hague insisted the UK will stand shoulder-to-shoulder with Gibraltar's people as he urged Spain to avoid increasing tensions further.
Mr Hague added a political solution is needed to ease the current issues with Spain, while David Cameron's spokesman said the Prime Minister was "seriously concerned".
The duo spoke after talk surfaced from Madrid that a 50 euro (£43.30) fee could be imposed on every vehicle entering or leaving the British Mediterranean outpost through its border with Spain.
Gibraltar's chief minister Fabian Picardo accused Madrid of "sabre-rattling" and behaving like North Korea following Spanish foreign minister Jose Garcia-Margallo's suggested border crossing levy.
Mr Garcia-Margallo said the proceeds could be used to help Spanish fishermen who have lost out because of damage to fishing grounds allegedly caused by Gibraltarian authorities. Such a fee could impose punitive costs on Gibraltarians who regularly commute into Spain to work.
Mr Picardo said "hell will freeze over" before the authorities in Gibraltar remove an artificial reef which Madrid claims is harming Spanish fishermen, adding that any border costs would violate European Union freedom of movement rules.
Spain is also considering closing its airspace to flights heading to the Rock. Mr Picardo claimed such a move would be dangerous and said it was the "politics of madness".
Mr Hague spoke with Mr Picardo on Monday and said he emphasised that the UK "stands shoulder-to-shoulder with the people of Gibraltar" at a time of "increasing Spanish pressure and rhetoric".
He said: "I also highlighted that we will respect Gibraltar's 2006 constitution and the commitments the UK has repeatedly made not to compromise on British sovereignty over Gibraltar. We discussed the need for a political solution to the current tension with Spain, which would be firmly in the interests of communities on both sides of Gibraltar's border with Spain.
"I call upon Spain to respect the agreements made at Cordoba and to avoid actions which could increase tension further. We agreed that it was important to respond to actions, not rhetoric, and I confirmed that we would continue to raise our concerns with Spain."