He said the rocky British exclave south of Spain had been "shamefully singled out", arguing that "exit negotiations clearly include Gibraltar despite Spain's attempts to also have Gibraltar excluded from them".
The nine-page roadmap introduced Friday by EU Council President Donald Tusk says that future Brexit agreements between the United Kingdom and the EU will only apply to Gibraltar if the governments of London and Madrid can agree to them.
Last year, Spain formally presented Britain with a proposal for joint sovereignty of Gibraltar - an offer that nearly everyone on the British overseas territory rejected.
In its opening negotiating position on Brexit, the European Union has said whatever deal is negotiated, it will not apply to Gibraltar without Spain's agreement.
Some 96% of Gibraltar's citizens voted to remain in the EU.
A Brexit deal will not automatically apply to Gibraltar and could be vetoed by Spain, according to the EU's draft guidelines for the UK's exit negotiations, reported the Independent on Friday.
Gibraltar's chief minister has told Sky News the territory "is not going to be a political pawn in Brexit" and will become even more British after leaving the union.
Gibraltar was taken from Spain in 1704 and ceded to Britain in 1713.
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Theresa May is coming under pressure to stand up to European Council proposals which would give Spain a veto on decisions over Gibraltar's future.
In 2002 a referendum which would have given Spain and Britain shared control of the region was rejected by nearly 99% of the population. The whole world and the whole European Union should know: "this changes nothing in respect of our continued, exclusive British Sovereignty".
Unless Theresa May is prepared to sacrifice the future of Gibraltar's 30,000 inhabitants to secure a good deal for the rest of the United Kingdom then she could have to make concessions to Spain.
"Theresa May must urgently produce a plan that protects the citizens of Gibraltar, including their businesses and communities".
"We have been firm in our commitment never to enter arrangements under which the people of Gibraltar would pass under the sovereignty of another state against their wishes, nor to enter into a process of sovereignty negotiations with which Gibraltar is not content", she added.
The Prime Minister told MPs the United Kingdom was clear that Gibraltar was "covered by our exit negotiations".
"Ministers should listen to the first minister of Gibraltar, who this week said that Gibraltar should be no bargaining chip in, pawn in or victim of Brexit", Labour lawmaker Mary Creagh said in a statement emailed by Open Britain, which campaigns to soften Brexit.
Andrew Rosindell of the All-Party Parliamentary Group For Gibraltar said "an agreement without including Gibraltar means there can be no agreement".