Boris Johnson and Downing Street have sought to reassure Gibraltar that the UK will stand up for the British overseas territory during the Brexit negotiations.
Gibraltar's government has accused Spain of trying to use Brexit to forward its territorial claims to the Rock, after an EU proposal to give Madrid a veto over decisions on the territory's future.
Prime Minister Theresa May has been criticised for failing to mention Gibraltar in her letter triggering Article 50, starting the Brexit process, while the negotiation guidelines set out by European Council president Donald Tusk would require Spanish agreement for the territory to be included in future agreements between the EU and UK on issues like trade.
Foreign Secretary Mr Johnson held talks with Gibraltar's chief minister Fabian Picardo in an effort to reassure him of the UK's continued support, while Downing Street highlighted comments made by Mrs May in her Commons statement on Brexit on Wednesday.
The Prime Minister told MPs that the territory was "covered by our exit negotiations" and vowed never to enter into talks over its sovereignty against the wishes of its people.
Mr Picardo said the territory was being singled out for "unnecessary, unjustified and unacceptable" discrimination as a result of "a disgraceful attempt by Spain to manipulate the European Council for its own narrow political interests".
Accusing Madrid of seeking to "mortgage the future relationship between the EU and Gibraltar to its usual obsession with our homeland", Mr Picardo said: "The whole world and the whole EU should know: this changes nothing in respect of our continued, exclusive British sovereignty."
After his talks with Mr Picardo, the Foreign Secretary said: "As ever, the UK remains implacable and rock-like in our support for Gibraltar."
Spain has a long-standing territorial claim on Gibraltar, which has been held by the UK since 1713 and has the status of British overseas territory.
Any suggestion Madrid might have a say over the future of the self-governing territory, which is home to important UK military bases, causes anxiety among its 30,000 inhabitants.
Gibraltar is addressed in a single paragraph of Mr Tusk's nine-page document, which says: "After the United Kingdom leaves the Union, no agreement between the EU and the United Kingdom may apply to the territory of Gibraltar without the agreement between the Kingdom of Spain and the United Kingdom."
Downing Street said Mrs May had made clear the Government was "absolutely steadfast in our support of Gibraltar, its people and its economy" in the House of Commons on Wednesday.
But Lord Boswell, chairman of the House of Lords EU Committee, said it was "unfortunate that the Prime Minister's letter to Donald Tusk, triggering Article 50, made no mention of Gibraltar - the Government mustn't give the impression that Gibraltar is an afterthought".
He said: "In the absence of any clear commitment to defend Gibraltar's interests by the Prime Minister, the door has been opened for the EU to present it as a disputed territory, without a voice of its own in negotiations that will have profound implications for its future prosperity."