Labour has placed itself squarely behind British citizens in Gibraltar and demanded that their rights be protected in the forthcoming Brexit negotiations. The intervention came as the territory’s leaders accused Spain of acting in a predatory manner over its future status.

The EU’s response to Britain’s article 50 letter formally announcing its departure singled out the status of Gibraltar, saying it could only be included in any deal between London and Brussels with Spain’s agreement.

On Saturday Labour’s shadow Brexit secretary, Keir Starmer, took to Twitter to warn about the status of the territory – which has voted repeatedly and overwhelmingly to remain British in recent referendums – after meeting his government counterpart David Davis on Saturday morning.

Gibraltar is not “a bargaining chip”, he said. In the meeting Starmer raised the concerns of British citizens in Gibraltar and “emphasised that it is vital that the sovereignty of Gibraltar is protected and that the interests of British citizens in Gibraltar are safeguarded”, a Labour spokesman said.

The government has been criticised for leaving the status of Gibraltar out of the article 50 letter to the EU, even though it is one of just three land borders that will be affected by Brexit.

Gibraltar’s chief minister Fabian Picardo denounced Spain’s position, and underlined the territory’s fierce opposition to Brexit, although he did not directly attack Theresa May and her government. “What we are seeing is a clear manifestation of the predictably predatory attitude that we anticipated Spain would seek to abusively impose on its partners,” he said.

“Gibraltar’s record as a member of the EU is an exemplary one and our people enthusiastically supported continued membership of the EU in the referendum … Brexit is already complicated enough without Spain trying to complicate it further.”

Liberal Democrats called on the government to draw up a plan to protect its citizens. “It is our obligation to support our overseas territories, and any attempt to brush off the importance of Gibraltar would be a dereliction of duty that would leave Margaret Thatcher spinning in her grave,” said Lib Dem foreign affairs spokesman Tom Brake.

The inclusion of Gibraltar as a high-profile negotiating pawn in Brexit talks with the EU reflects the rise of Spain within the bloc as Britain prepares to leave, said Emilio Sáenz-Francés, professor of international relations at Comillas University in Madrid.

“The departure of the UK brings Spain into the first ranks of the EU, and in some way it [the EU] is recognising this with the veto,” he said. “[Spain] has been unofficially added to the club of the big countries.”

A showdown over Gibraltar could be painful for the Spanish government at a time of economic stagnation, with areas near the Rock heavily dependent on the jobs it provides.

But Brexit hardliners in the Spanish government may be willing to endure those political costs in a bid to take advantage of a rare opening.

“Spain wants to use Brexit as leverage. It’s one of the moments in history where there’s a change in the background conditions that allows you to push for something,” said Antonio Barroso, an analyst at the political risk advisory firm Teneo Intelligence. “The ultimate objective is co-sovereignty.”

Additional reporting by Sam Jones in Madrid