The UK reached an agreement with the EU on Friday, conceding grounds like fishing and immigration to get the deal finished quickly.
But Spanish PM Mariano Rajoy has put it in at risk by requesting that a footnote referencing their veto is put into “green” text, which indicates it is agreed by all sides.
An EU diplomat said: “Spain is playing hardball.”
The move prompted EU Council chief Donald Tusk to write to leaders telling them securing an agreement on the pact “remains open”.
He said: “I still need a couple more hours to consult with some of the most concerned Member States.”
On Monday Michel Barnier and David Davis published a colour-coded chart showing progress in different areas of talks.
Green covered 75 per cent of the text and indicated signed off passages with yellow meaning partial agreement and white areas of divergence.
None of the footnotes were coloured in although one EU official said the asterisk linking Spain’s demand is shaded green.
The official played down the risk of Spain’s demands and said President Tusk hoped to be able to iron out any differences.
He said: “Before Tusk can recommend to the leaders ‘please do welcome that agreement including on transition’ Member States should have a chance to look at it.”
Bulgarian minister Ekaterina Zaharieva said the bloc would show “solidarity with Spain”.
The move comes amid more divisions between countries.
Brussels has seemed to discount Philip Hammond’s hopes of a special financial services deal in the latest version of its draft negotiating guidelines.
The new positions seem to rule out Mr Hammond’s plan for a new system of “mutual recognition” of legal standards.
Brussels demands a more flimsier relationship based on “equivalence” that Brussels could scupper at any moment.
But the statement has not been agreed by all 27 member states.
The dossier, which has been seen by The Sun, says: “Regarding financial services, the aim should be reviewed and improved equivalence mechanisms, allowing appropriate access to financial services markets, while preserving financial stability, the integrity of the single market and the autonomy of decision making in the European Union.
“Equivalence mechanisms and decisions remain defined and implemented on a unilateral basis by the European Union.”
Gibraltar’s chief minister Fabian Picardo shut down Madrid’s calls for more control over the region’s airport once the British territory leaves the European Union after Brexit.
Mr Picardo warned Madrid that Gibraltar “will not compromise” its sovereignty or jurisdiction over Gibraltar International Airport once Brexit becomes official in 2019.
Officials from both sides have been holding talks over the future of the airport as a way to improve relations during and after Brexit.
But the chief minister dismissed calls for shared management of the airport, suggesting that Spain could seek “enhanced use” of it instead.
Mr Picardo said: “Let’s be very clear about the issue of the airport and indeed all other aspects of this negotiations.
“We will want to continue to offer an olive branch of cooperation and friendship to the people of Spain as we have at every stage but that doesn’t mean we will make concessions on sovereignty, jurisdiction or control.”