The Hong Kong authorities have usually acceded to extradition requests under a 1997 treaty with the US. But the final say could rest with China, which assumed sovereignty later that year.

Edward Snowden has said he chose Hong Kong because of the former colony’s “strong tradition of free speech”. But that dates back to the days when it was a haven for political dissidents from China.

The bilateral treaty with the US makes clear that China – should it choose to do so – could lean on the territory to block any extradition request. Beijing itself does not have an extradition agreement with Washington, and Article 3 of the US-Hong Kong treaty allows the latter not to hand a person over if it believes that it might impact on China’s “defence, foreign affairs or essential public interest or policy”.

Hong Kong authorities can also refuse extradition if either it or China starts its own prosecution, or believes the request is politically motivated, and that the person would be prosecuted for his political views. As a result Washington may choose not to pursue Mr Snowden under the Espionage Act, but instead indict him under a different crime listed on the US-Hong Kong agreement, which includes “the unlawful use of computers”.

According to the South China Morning Post, Mr Snowden’s best chance of staying in Hong Kong would be to apply for refugee status. To achieve this, he would have to claim he would be exposed to “cruel, inhuman or degrading treatment or punishment”.