The EU has handed Spain an effective veto over the entire Brexit deal by declaring Madrid must agree to it applying to Gibraltar.
It means Theresa May could be faced with choosing between accepting a deal that leaves the Rock in the wilderness or walking away from the talks with nothing.
Downing Street tonight said Mrs May had already been clear in her support for the Rock and its people.
And Gibraltar itself blasted the 'disgraceful' singling out of the 30,000 residents by the European Union.
MPs were outraged at the move, contained in the small print of the draft negotiating guidelines for the EU published by Council President Donald Tusk today.
Gibraltar's chief minister Fabian Picardo said: 'This unnecessary, unjustified and unacceptable discriminatory proposed singling out of Gibraltar and its people was the predictable machination of Spain that the people of Gibraltar foresaw and one of the reasons we voted so massively to remain in the EU.
'This is a disgraceful attempt by Spain to manipulate the the European Council for its own narrow, political interests.
'Brexit is already complicated enough without Spain trying to complicate it further.'
Downing Street said Mrs May had made clear the Government was 'absolutely steadfast in our support of Gibraltar, its people and its economy' in her Commons statement on Wednesday.
The Prime Minister told MPs the UK was clear that Gibraltar was 'covered by our exit negotiations'.
'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 said.
Spain defended the position today as minister Inigo Mendez de Vigo said 'this is an important point in the future negotiations of the United Kingdom's exit from the EU'.
A senior diplomat in Brussels said: 'This seems intended to give Spain something so they don't try to hold the whole withdrawal treaty hostage over it.'
Andrew Rosindell, the Vice Chair of the All-Party Parliamentary Group for Gibraltar, told the Telegraph: 'An agreement without including Gibraltar means there can be no agreement.
'British people must and will stand together, we cannot be bullied by Spain, any agreement must apply equally to the whole British family and that includes Gibraltar.
'There can be no compromise on this.'
Conservative MP Bob Neill said: 'Gibraltar's friends in the UK will be watching this very carefully. There will be no sell out.'