LONDON (Reuters) - British Foreign Secretary Boris Johnson has said the United Kingdom would remain “implacable and rock-like” in its support for Gibraltar after the European Union offered Spain a right of veto over the territory’s future relationship with the bloc.

Often referred to as “The Rock”, Gibraltar is a British overseas territory on the southern tip of Spain.

Madrid claims sovereignty over the territory -- which it ceded to Britain in 1713 -- and the issue has often caused diplomatic tensions with London.

Gibraltar’s status after Britain leaves the EU is set to be one of the most contentious issues in complex exit talks expected to last years.

Johnson made his comment about Gibraltar late on Friday after speaking on the phone to Fabian Picardo, Gibraltar’s chief minister.

“As ever, the UK remains implacable and rock-like in our support for Gibraltar,” he wrote on Twitter.

Picardo has accused Spain of a “disgraceful” attempt to manipulate the Brexit process to further its sovereignty claim over Gibraltar.

According to the EU’s draft joint position on exit talks with Britain, no agreement between the UK and the EU may apply to Gibraltar without a separate agreement between Spain and the Britain.

The Spanish government said it was satisfied with the position.