DW: Why is the US opposed to a Brexit and says so publicly?

Xenia Wickett: There are three principal reasons why the UK is such an important special ally of the US. Because of the assets it brings to the table, the capabilities, intelligence and military. Because of the close and common perspective and the fact that the UK is often a partner and ensures that the US doesn't have to act alone. And finally, because the UK is the closest thing the US has to a voice in Europe. As someone who thinks similar and has similar interests it can transmit or at least follow American interests in Europe; if the UK is no longer in Europe that is no longer credible. At the same time the US also thinks that as far as the European Union is concerned, as so many do, there would advantages to a reform, and the UK is the most likely state to push for that reform.

How then would Britain leaving the EU affect the traditionally close relationship between the US and Britain?

You are already seeing the US diversifying its relationships into continental Europe: If is talking about Russia, it calls Merkel; if it is talking about economics, it calls Chancellor Merkel; if it is interested in anti-terrorism activities in parts of Africa, it calls Francois Hollande. Who were the first actors to step up in Syria? That was the French, not the Brits. So this relationship is already beginning to diversify. That diversification will be hastened. What has long been a special relationship - that is being based both on common values, but also a common ability to meet mutual interests - is increasingly no longer the case. The US is looking elsewhere. If Brexit happens that will be hastened, and the demise of the special relationship will be hastened.

How much weight does Washington advocating in public for Britain remaining in the EU have in the country?

President Obama is going to have to be very careful, as he and others in the US administration already have been. The US has no right to suggest to the British what they do with their own sovereign decisions. The US absolutely has a responsibility to explain what their interests are. And so what President Obama will do is explain American interests in the UK staying in Europe. But he will not, I believe, talk about what he believes would be the right thing for the UK to do. There is a certain amount of polling that suggests that there is enormous respect for Obama here in the UK, that President Obama is respected here and is listened to. And that is also clear for the Brexit team, who are absolutely determined to try and prevent him saying anything, because they do believe that it would be influential.

Xenia Wickett heads the US and the Americas Program at Chatham House in London where she is also the dean of the Queen Elizabeth II Academy for Leadership in International Affairs.