The shadow Brexit secretary says there is a "vital need to protect (the) sovereignty and interests of Gibraltar".
Amid fears of an EU ambush over the territory's future after the UK triggered Article 50, Labour's Sir Keir Starmer has revealed he has held talks with the Brexit Secretary.
"Spoke to Brexit Secretary David Davis this morning about vital need to protect sovereignty and interests of Gibraltar," the shadow Brexit secretary tweeted. "Not a bargaining chip."
The heat is being turned up on the Government after criticism of Theresa May for failing to mention Gibraltar in her Article 50 letter to European Council president Donald Tusk.
At the same time, negotiation guidelines set out by Mr Tusk appear to put the future of Gibraltar at stake by giving Spain a veto in discussions about the Rock's future.
"After the UK leaves the Union, no agreement between the EU and the United Kingdom may apply to the territory of Gibraltar without the agreement between the Kingdom of Spain and the United Kingdom," he wrote.
Revealing the discussion with Mr Davis, a spokesman for Sir Keir said the shadow Brexit secretary "raised the understandable concerns of British citizens in Gibraltar".
Sir Keir was also said to have emphasised that it was "vital that the sovereignty of Gibraltar is protected and that the interests of British citizens in Gibraltar are safeguarded".
After the "veto" row broke, Foreign Secretary Boris Johnson spoke on Friday with Gibraltar's chief minister Fabian Picardo in a bid to reassure him of the UK's continued support.
And after his talks with Mr Picardo, the Foreign Secretary tweeted: "As ever, the UK remains implacable and rock-like in our support for Gibraltar."
He said that Spain was "employing her unhealthy obsession with Gibraltar and bringing it to the table of a very complex negotiation already".
Complaining that the Tusk guidelines would allow Spain "to discriminate against the British people of Gibraltar", he said: "Gibraltar is not going to be a political pawn in Brexit. Neither is it going to be a victim of Brexit.
"Gibraltar is going to be very prosperous, very successful and entirely British before, during and after Brexit."
Describing its future "as even more British after Brexit than it is now", he said: "The whole of Europe will see that Spain is trying to abuse this moment for her own selfish political purposes."
Mr Picardo said the territory was being singled out for "unnecessary, unjustified and unacceptable" discrimination as a result of "a disgraceful attempt by Spain to manipulate the European Council for its own narrow political interests".
And accusing Madrid of seeking to "mortgage the future relationship between the EU and Gibraltar to its usual obsession with our homeland", Mr Picardo added: "The whole world and the whole EU should know: this changes nothing in respect of our continued, exclusive British sovereignty."
Downing Street has said the Prime Minister made clear in her Commons statement on Article 50 on Wednesday that the Government was "absolutely steadfast in our support of Gibraltar, its people and its economy".
But former Tory MP Lord Boswell, chairman of the House of Lords EU Committee, has said it was "unfortunate that the Prime Minister's letter to Donald Tusk, triggering Article 50, made no mention of Gibraltar".
He said that "in the absence of any clear commitment to defend Gibraltar's interests by the Prime Minister, the door has been opened for the EU to present it as a disputed territory".
Clare Moody, Labour MEP for the South West and Gibraltar, said it was a "shock" that the Rock was not mentioned in Mrs May's Article 50 letter.
"It is the UK Government's job to represent the people of Gibraltar, and I was amazed that they failed to do that in the letter that they sent in on Wednesday," she said.
And Green Party MP Molly Scott Cato said: "The people of Gibraltar voted 96% to Remain on an 85% turnout yet are being dragged, against their will, into the Tories' Brexit shambles.
"They now face the prospect of becoming pawns in a government showdown between Tories pushing for a new trade deal outside the single market and those who believe that the sovereignty of the UK overseas territory is non-negotiable.
"This shows the irresponsibility of pushing for a hard Brexit for which the government have no mandate, and certainly one the people of Gibraltar have no desire for."
Pro-Gibraltar Tory MP Andrew Rosindell told Sky News the Government must stand firm against the Spanish threat.
"Any Brexit agreement without including Gibraltar means there can be no agreement," he said.
Any deal must apply equally to the whole British family and that includes Gibraltar. There can be no compromise on this."