With talk of a backbench coup against the British prime minister fizzling out, No 10 said May would head for discussions with Juncker “as part of the ongoing negotiations over the future framework”.
Downing Street and the European commission have a deal in principle on the withdrawal agreement dealing with citizens’ rights, the £39bn divorce bill and the Irish border problem. There are continuing negotiations on a separate document about the future.
May’s trip comes amid a slight easing of recent pressure on her position, with speculation about an imminent leadership bid fading as Brexiter rebels failed to amass the 48 letters of no confidence from Conservative MPs needed to spark a challenge.
She does, however, face a parallel problem after the DUP, the party keeping her in office, breached their so-called confidence and supply dealwith the Tories by failing to back the government on a series of budget votes on Monday night.
Earlier, the Spanish foreign minister, Josep Borrell, confirmed that May would visit the Belgian capital within 24 hours to sign off on the joint political declaration on the future relationship.
Borrell and his prime minister, Pedro Sánchez, warned, however, that Spain could withhold its support for the entire package unless there was an explicit statement that a future trade deal with the UK would only cover Gibraltar if it received Madrid’s consent.
“In the past 72 hours none of the documents have clarified something which is fundamental for us,” Sánchez said on Tuesday morning. “Gibraltar is not part of the UK … As it stands, the [Spanish] government will vote against the Brexit agreement.”
Borrell echoed his prime minister, adding: “Mrs May said that she will not agree with the withdrawal treaty ... without knowing the content of the political declaration about the future relationship.
“She has said she will not agree with one thing unless I know and agree with the other. Me too. I want to see both things. Maybe tomorrow. Mrs May is coming here.”
Borrell said that the EU’s chief negotiator, Michel Barnier, was “trying to write a paper that fits everything together and makes [everybody] happy”.
But he said there were some things that did not make the Spanish very happy, adding: “We have always been working with one idea, an idea approved several times by the European council. On the future relations, it will not include Gibraltar. Gibraltar is something apart ... Spain has to agree … we want it to be clear one way or another.
“I don’t mind about the instrument but I want it to be clear the future will be negotiated about Gibraltar with the agreement of Spain.”
Borrell said the withdrawal agreement would still go through without Spain’s support, as it did not require unanimity.
He added, however, that the Brexit process had laid bare the fragility of the UK, and he warned that he could foresee Scotland becoming independent and joining the EU.
“Why not?”, Borrell told Politico in an interview on Tuesday morning. “If they leave Britain within the accordance of the internal regulations. If Westminster agrees, we are not going to be more Papist than the pope. If Westminster agrees why would we be against it.”
Despite the recent crisis in Catalonia sparked by the independence movement, Borrell added: “I am very much [more] worried about the unity of the United Kingdom than the unity of Spain. I think the United Kingdom will split apart before Spain. That is a clear sign of the United Kingdom’s weakness.”
He warned against a second Brexit referendum, as proposed by his own prime minister last week, however.
Borrell said: “My prime minister the other day, said that one way out of this story would be to hold a second referendum, why not? But I don’t like second referendums. In Europe, we have a record of people voting, voting something that was not what the authorities liked and then being convinced to vote again. It was the case with the Irish at least twice and the Danish also. It is also something that doesn’t look very democratic.”
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