British lawmakers have been working on a EU Referendum Bill as part of the legal preparations for the proposed referendum.

In the interview, Obama admitted the "most frustrated" area during his presidency was failure of gun control "even in the face of repeated mass killings".

"If you look at the number of Americans killed since 9/11 by terrorism, it's less than 100. If you look at the number that have been killed by gun violence, it's in the tens of thousands," Obama said.

"For us not to be able to resolve that issue has been something that is distressing," he told BBC.

The United States and Britain have enjoyed a so-called "special relationship" after the WWII.

However, political analysts note that the "special relationship" has become increasingly irrelevant, as Britain has been more willing to take a back seat in global affairs following the global financial crisis.