GIBRALTAR - Negotiations over the post-Brexit future of Gibraltar are deadlocked on the issue of security at the territory’s airport, raising fears of a deepening stand-off between Britain and Spain on the contentious issue of The Rock. Sources on the UK and Gibraltar side of the negotiations said that Spain had touched a “red line” with its demands to have police stationed at the airport which is partially situated on a disputed isthmus that links the territory with mainland Spain. “The Spanish crossed a red line,” said a source with knowledge of the talks which started this year in a bid to avoid a last-minute stand-off over Gibraltar. “They want to get their Guardia Civil and police into Gibraltar…"