Bobbing on the port captain's launch off the coast of Gibraltar on Tuesday there was no evidence of how this calm stretch of sea could have caused such an international storm. A handful of craft fishermen cast their nets, seemingly oblivious to the fact that 10 metres below their dinghies lay the unlikely catalyst for a political row that has embroiled David Cameron in the UK's bitterest battle with Spain over "the Rock" since Franco.

Here, late last month, Gibraltar dumped an artificial reef on a fishing ground favoured by Spanish scallop dredgers. Now the ripples from those dozens of concrete blocks are rocking a 300 year old British enclave that for some is an emblematic imperial redoubt and for others an awkward colonial hangover.

On Tuesday Gibraltar said it was preparing for legal action against Madrid over its retaliation for the reef, which has taken the form of a frontier control go-slow that has caused residents to queue for up to six hours in scorching summer temperatures. Gibraltar has accused Spain of inhumane behaviour and is gathering evidence that could be used at the European Court of Human Rights....