Spanish prime minister Pedro Sanchez demanded assurances that the future of the disputed British territory of Gibraltar would be settled through direct talks with Madrid.
Mr Sanchez said: "In the past 72 hours, none of the documents has clarified something which is fundamental for us - Gibraltar is not part of the UK".
Madrid has raised objections to wording in the agreement about Gibraltar, the tiny territory at the tip of the Iberian Peninsula that was ceded to Britain in 1713 but is still claimed by Spain.
Spain's foreign minister, Josep Borrell, told Politico today that it was the fault of EU negotiators, who neglected to include text already agreed between the United Kingdom and Spain despite multiple previous confirmations with European officials.
Gibraltar chief minister Fabian Picardo said he has received assurances from Theresa May that the deal will cover "the whole United Kingdom family". "As a effect, today, I regret to say that a pro-European government like Spain's would vote no to Brexit unless there are changes".
But Mr Borrell dismissed the criticism and said all the agreements relating to Gibraltar were now in place, adding that Spain's only concern was in relation to negotiations for the future relationship.
May's official spokesman said: "The draft withdrawal agreement agreed last week covers Gibraltar".
Spain threatens Brexit deal over Gibraltar status
European Union officials say they believe the Gibraltar issue can be solved before European Union leaders sign the withdrawal agreement at a summit on Sunday.
According to the BBC, Spain's government now wants the words "This does not apply to Gibraltar, which will be subject to bilateral talks between the United Kingdom and Spain" added to the 585-page withdrawl agreement document.
Separately, Madrid and London reached four agreements governing the relationship between Spain and Gibraltar after Brexit covering tobacco smuggling, police cooperation, the rights of cross-border workers, the environment and one tax agreement, the Spanish government said Wednesday.
Since Britain is leaving the European Union, and Spain is staying in, the other 26 member states have no incentive to take Britain's side or even get involved.
The UK Government has made clear it will not exclude Gibraltar from discussions on the future arrangements after withdrawal, a message repeated in the House of Commons by the Prime Minister yesterday.
Jeremy Corbyn said it would not have the backing of Labour, meaning May lacks enough Parliamentary backers to get the deal through.
The announcement stands in contrast from Spain's previous position which was to veto Scotland's attempts to join the European Union so as to dissuade Catalonian separatists.