Britain's support for Gibraltar will remain "implacable and rock-like", foreign secretary Boris Johnson has promised.
The statement came late last night after the European Union said Spain would be given the final say on how the deal affects the British overseas territory.
Located on the southern tip of Spain, rule over the territory has been held by Britain since 1713, but Madrid claims sovereignty.
After speaking on the phone with Gibraltar's chief minister, Fabian Picardo, Johnson tweeted "As ever, the UK remains implacable & rock-like in our support for Gibraltar."
Picardo previously accused Spain of using the Brexit process to push forward with its sovereignty claim.
Clare Moody, Labour MEP for Gibraltar and south west England, told BBC's Today programme said:
"It worries me that we are about to enter into the most detailed negotiations that we have known for decades.
"If the government has overlooked the interests of Gibraltar, which is a crucial part of the kind of constitutional arrangements of our membership of the European Union, then what else are they going to overlook as well?"
Although it voted overwhelmingly in favour of remaining in the EU, Picardo said Gibraltar is not willing to become sovereign to Spain just to stay in the EU.