Spain’s foreign ministry on Friday condemned the attack on the customs agents, who had suffered “serious injuries” and said it “categorically rejects the terms” of the statement issued by Gibraltar, “as well as the claims of alleged British sovereignty over the territory and waters of Gibraltar contained within it”.

The incident comes as Britain and Gibraltar seek to negotiate a treaty to settle Gibraltar’s post-Brexit status and how to police the border with Spain.

This has been a point of contention since Britain’s 2016 vote to leave the European Union. The peninsula was excluded from the exit deal reached between Britain and the EU.

In mid-January, Spain’s Foreign Affairs Minister Jose Manuel Albares told Reuters Madrid and London were “very close to a deal” on the issue.

Spain ceded the rocky outpost at the mouth of the Mediterranean Sea to Britain in 1713 but has long called for it to be returned. In 2002, 99 per cent of voters in Gibraltar rejected the idea of Britain sharing sovereignty with Spain.