Downing Street, which has condemned the move, nevertheless admitted Ms May had previously warned that Spain could exploit any attempt to renegotiate the withdrawal agreement.
“The prime minister has previously cited, in the Commons that, if the withdrawal agreement is reopened, others may seek to raise these issues,” her spokesman told The Independent.
The 300-year Anglo-Spanish dispute over Gibraltar has blown up again after the EU agreed to a footnote to a proposed EU law to provide visa-free access to UK nationals if the UK leaves without a deal.
“Gibraltar is a colony of the British crown,” said the text, approved by the European Council of the 27 EU members.
“There is a controversy between Spain and the UK concerning the sovereignty over Gibraltar, a territory for which a solution has to be reached in light of the relevant resolutions and decisions of the general assembly of the United Nations.”
Ben Bradshaw MP, a Labour supporter of the People’s Vote campaign, said: “This latest row over the sovereignty of Gibraltar is an inevitable consequence of Theresa May’s attempts to reopen the withdrawal agreement simply to appease Brexiters.
“The people of Gibraltar overwhelmingly rejected Brexit in 2016 but are having every aspect of their future threatened by it.”
Mr Bradshaw said the episode strengthened the case for a Final Say referendum, rather than the prime minister “embarking on a further round of fantasy Brexit”.
No 10 hit out fiercely at the text adopted in Brussels, the spokesman saying: “It is completely unacceptable to describe Gibraltar in this way.
“Gibraltar is a full part of the UK family. This will not change due to our exit from the EU.”
However, the spat underlined how the dispute over the territory could still pose a serious obstacle to an orderly Brexit.
Spain, which pressed its demands on Gibraltar’s status in the run-up to the agreement being signed in December, will expect the new footnote to be used in all future EU legislation regarding the UK after its departure.
The UK and Gibraltar governments argue that the territory has a right to self-determination. In a 2002 referendum on sharing sovereignty with Spain, 98.9 per cent of Gibraltar voters rejected the proposal.
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