On 8 June 1969 Spain ordered the closure of the customs post and border gates between Spain and Gibraltar at La Línea. On 25 June the ferry service from Algeciras to Gibraltar was suspended with effect from 27 June, leaving the weekly BEA flight from London via Madrid as the only communication between Spain and the Rock. So began the fifteenth and longest siege of Gibraltar.

The British Foreign Secretary Michael Stewart commented in the House of Commons on 26 June that the interruption of the ferry service not only flouted ‘standards of international behaviour accepted by modern governments’, but also that it was in breach of the Treaty of Utrecht, which implies that there should be sea communication between Spain and Gibraltar. He accused Spain of subjecting the Gibraltarians to a policy of economic and psychological pressure. In an aide-mémoire handed to the Spanish Ambassador in London when he was summoned to the Foreign Office on 27 June, the Foreign Secretary said, prophetically: ‘… a policy of deliberate hostility against the inhabitants of Gibraltar can only be self-defeating … Her Majesty's Government find it impossible to understand what interests can be well served by severing all means of surface communications between two closely-linked communities …’.

If General Franco and, more particularly, his hard-line Foreign Minister Sr Castiella thought that a policy of isolating Gibraltar would both bring Gibraltar to its knees economically and result in the inevitable transfer of Gibraltar to Spain as the only path to its survival, they were clearly very much mistaken on both counts.