Spain could potentially veto a UK trade deal with the EU if it does not get a say over the future of Gibraltar.

Conservative MPs have warned the sovereignty of the UK overseas territory is non-negotiable.

Spain’s potential veto was contained in a leaked draft document setting out the European Union’s Brexit negotiating strategy.

It said no agreement on the EU’s future relationship with the UK would apply to Gibraltar without the consent of Spain.

In response, Conservative MP Jack Lopresti said Spain was using Brexit as “a “fig leaf for trouble making” while fellow Tory Bob Neill tweeted “no sell out”.

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Spain has long contested Britain’s 300 year-rule of Gibraltar but Gibraltarians rejected the idea of the UK sharing control with Spain in a vote in 2002.

In a referendum, the approximately 30,000 residents of “The Rock” voted by 99% to 1% to reject the concept of dual sovereignty.

However, this did not put an end to the issue, with Spain continuing to press its territorial claim and periodic incidents heightening tensions between the two countries.

In the wake of June’s referendum – in which Gibraltar voted by a margin of 96% to 4% to remain in the EU – Spain’s then foreign minister José Manuel García-Margallo suggested shared sovereignty could allow Gibraltarians to maintain some of the benefits of EU membership and enable Spain to “plant its flag” there.

‘Bullied’

But Alfonso Dastis, who replaced Mr García-Margallo, struck a more conciliatory tone in January.

He said Spain would not put Gibraltar – which joined the EU at the same time as the UK in 1973 – at the centre of the negotiations and Gibraltar would be free to leave the EU if it wishes.

In its draft Brexit negotiating guidelines – which have yet to be approved by all member states – the European Council identified future arrangements for Gibraltar as one of its 26 core principles.

Gibraltar: key facts

• Gibraltar is a British Overseas Territory although Spain, which it borders, also claims sovereignty
• Gibraltarians are British citizens but they run their own affairs under a chief minister
• The territory is self-governing in all matters – including taxation – except foreign policy and defence, which are dealt with by the British government
• Despite its small size, Gibraltar is strategically important, standing at the mouth of the Mediterranean only 12 miles from the north coast of Africa. It has a UK military base, including a port and airstrip

It wrote: “After the UK leaves the union, no agreement between the EU and the UK may apply to the territory of Gibraltar without agreement between Spain and the UK.”

Brussels officials were quoted by the Guardian as saying the EU was standing up for its members interests.

“That means Spain now,” a senior EU official told the newspaper. “The text means what it says. Any extension of the deal [after withdrawal] to Gibraltar, which is a disputed territory, will require the support of Spain.

“It recognises that there are two parties to this dispute.”

‘Shameful’

Gibraltar’s inclusion in the document met with an angry reaction from Conservative MPs.

Mr Lopresti, chair of the All-Party Parliamentary Group on Gibraltar, said there was no question of any negotiation over its future and he would be raising the matter with the secretary general of Nato, of which the UK and Spain are both members.

“It is predictable that given Spain’s previous behaviour, they would try and use Brexit as a fig leaf for trouble-making over the status of Gibraltar,” he said.

“It is shameful that the EU have attempted to allow Spain an effective veto over the future of British sovereign territory, flying in the face of the will of the people of Gibraltar.”

Border controls

And Mr Neill, chair of the Commons Justice Select Committee – which examines relations between the UK and its overseas territories – said it was a matter of concern.

He tweeted: “Gibraltar’s friends in the UK will be watching this very carefully. There will be no sell out.”

Labour MP Mary Creagh, a supporter of the Open Britain campaign group, said Gibraltarians risked being treated as “pawns” in the Brexit process.

“The Rock depends on free movement of labour from Spain, and on its place in the single market to support its vital services industries,” she said.

“‘Brextremists’ should be ashamed that their actions have destabilised the situation in Gibraltar.”

Gibraltar’s government has ruled out any dilution of sovereignty in return for continued access to the European single market or other benefits attached to EU membership.

Key issues in post-Brexit negotiations relating to Gibraltar are likely to be border controls – thousands of people commute in and out of the territory from the mainland to work every day – and airport landing rights.