Britain is only two days away from voting on whether to leave the European Union.
The upcoming national referendum has resulted in a divisive "Brexit" debate, with polls indicating an extremely close decision.
President Barack Obama, who visited London in April, has warned that the outcome of Thursday's referendum is "of deep interest to the United States" and could indicate a turning point for Europe.
Britain is one of the most important members of the EU, is the EU's second largest economy and has a powerful military. Some observers fear that Britain leaving the EU will weaken America's allies and bring instability.