The treaty was signed in 2003. Between January 1, 2004, and yesterday, 92 people have been extradited from the UK to the US. In the same period, 43 people have been extradited from the US to the UK.
How does the process of extraditing someone from the US to the UK work?
A request is sent to the US Start Department via the British Embassy in Washington. There are two tests which must be met. There must be information that would provide "reasonable basis to believe that the person sought committed the offence".
It is also examined by the Department of Justice, to ensure that there is "probable cause" to justify it. This test comes from the Fourth Amendment to the US Constitution and suspects can challenge the lawfulness of their arrest.
The Secretary of State has the final decision after considering human rights issues.
How does it work from the UK to US?
They have less stringent requirements – leading to criticisms that the treaty is one-sided. Proceedings begin with a US prosecutor issuing an arrest warrant, which is passed onto the Crown Prosecution Service.
UK Police arrest the suspect, and courts decide if technical requirements for extradition have been met. The US does not need to provide all the evidence that would be required in a full trial before a British court.
Why is the treaty so controversial?
It allows British suspects to be extradited to a foreign country to stand trial on evidence which would fall far short of what would convict them in their home country.
Does it matter if a crime is not committed on US soil?
The treaty makes no distinction where the crime was committed, only that US laws were contravened.
What about a person's human rights?
Civil liberties groups have raised fears about UK citizens' human rights under the treaty, saying those extradited the US may face years in solitary confinement in a high-security prison.