Gibraltar-loving Tory MPs have been accused by its chief minister of putting their Euroscepticism before his people's best interests by backing Brexit.
In a sharp caution to long-term allies to think again, Fabian Picardo suggested it was not "compatible" to be a "real friend" of the overseas territory and vote to force it out of the EU against its will.
He also accused the Government of leaving The Rock in the lurch by refusing to conduct any contingency planning in the event of a Leave vote in the referendum.
Mr Picardo is pushing to highlight to UK voters what he believes would be the disastrous consequences for his country of withdrawal from the EU, with Spain "ready to pounce".
Its 22,000 residents - who are British citizens - have a vote in the referendum and are expected with few exceptions to back Remain amid fears Madrid could close its vital border as part of a new drive to reclaim sovereignty.
The chief minister told the Press Association there was "no doubt that we would be worse off outside the European Union" and he was "not in the business of risking Gibraltar's continued economic prosperity".
Tory MP Andrew Rosindell is among the most vocal defenders of the territory's right to remain part of the UK in the face of Spanish claims and a prominent member of the Conservative Friends of Gibraltar.
But he is one of several members of the all-party group on Gibraltar - including its Tory chairman Jack Lopresti and Democratic Unionist Ian Paisley - who intend to back a Brexit on June 23.
Mr Picardo questioned their commitment, insisting vital help from the European Commission in forcing Madrid to respect free movement at the border "would go out of the window" if the Eurosceptics won the argument.
"I'd say the majority of the group is in favour of staying in," he told PA.
"But we do have people who are very good friends of Gibraltar and who have long argued that they would like to see the UK out of the European Union.
"I ask them to think very carefully about what they are doing and how that could reflect on Gibraltar. To such a extent that I wonder if it is compatible to be in favour of Brexit and call yourself a real friend of Gibraltar. I say that advisedly.
"I know that Andrew and others have been great friends of Gibraltar and I know that they feel great friends of Gibraltar. We consider them friends too.
"But the consequences of Brexit are so potentially dangerous to Gibraltar and the risks to which we will be exposed are so huge that I think if you really put on your thinking cap for Gibraltar, you would only come to the conclusion that Brexit is an unnecessary risk to take.
"They say that they are sure that the United Kingdom would defend Gibraltar etc etc. But it is difficult when you ask people 'how?' and to say more about what they would do, to see that they have no answers other than pure speculation."
Mr Picardo said he was lobbying Europe minister David Lidington to drop a Whitehall ban on contingency planning to allow discussions on issues such as whether Gibraltar would be included in any post-Brexit trade deals.
"The UK Government is not planning for an exit," Mr Picardo said.
"I would certainly rather it was because frankly I think we need to be ready for all eventualities.
"Their instructions are that they cannot at this stage explore what might happen in the event of an exit because the position of the Government is to support staying in."
He said his government was looking at how to minimise the potential damage.
"The fact that the result of the referendum is not in yet is not going to stop us trying to consider all of those issues," he continued.
"Even if we are not issuing press releases on the subject at this stage, we need to understand what we could be able to do in the context of that new reality.
"If the United Kingdom were negotiating a trade deal with China, why should Gibraltar be included with it if the rest of the overseas territories are not? Ditto the United States for example.
"For Gibraltar you can take it as a given that we would find it very difficult indeed to negotiate bilateral trade deals with the United States or Canada or any other that the trading bloc of the UK might be able to do deals with.
"Our current economic model is based on the ability to freely provide services in the single market.
"Therefore the proposal to leave the single market, without clarity as to what access we might have or might be able to agree, is very worrying indeed.
"It is all in the realms of speculation and that therefore involves risk and I am not in the business of risking Gibraltar's continued economic prosperity.
"I have no doubt that we would be worse off outside the European Union because our current prosperity has demonstrated just what membership of the EU can mean."
Mr Picardo said he feared pro-Brexit voters were more motivated to get to the ballot box and urged the Conservative and Labour leaderships to step up their efforts to enthuse others to turn up to tick the Remain box.
"Those who want to remain within the European Union should exercise their right to vote because those who are going to vote for the United Kingdom to be outside the EU are certainly going to be ensuring that they have set aside June 23 as a day that they need to be involved in voting," he said.
"I am convinced that with all parties demonstrating to their membership how important the European Union can be going forward, the result should be a clear majority for the 'in' campaign.
"But the political leaders have a responsibility to make the argument.
"The Labour Party need to champion membership of the European Union as something that is important to working people as much as the Conservative Party needs to champion membership of the European Union is important to business."