Britain's decision to quit the European Union could send damaging shock waves through the bedrock Anglo-American "special relationship," raising questions about London's willingness and ability to back U.S.-led efforts in global crises ranging from the Middle East to Ukraine.

The loss of the strongest pro-U.S. voice within the 28-nation bloc, as a result of the Brexit referendum, threatens to weaken Washington's influence in European policymaking and embolden Russian President Vladimir Putin to further challenge the West, analysts and former diplomats say.

The British referendum held Thursday, widely seen as reflecting an increasingly nationalistic and inward-looking public, also risks the splintering of the United Kingdom itself, which could further reduce its role and stature in world affairs.