HONG KONG (Reuters) - A former U.S. National Security Agency (NSA) contractor charged with spying by the United States and in hiding in Hong Kong is expected to be the subject of a formal extradition request at any time in what could drag into a legal battle lasting years. Since making his revelations about massive U.S

HONG KONG (Reuters) - A former U.S. National Security Agency (NSA) contractor charged with spying by the United States and in hiding in Hong Kong is expected to be the subject of a formal extradition request at any time in what could drag into a legal battle lasting years.

Since making his revelations about massive U.S. surveillance programmes, legal sources in Hong Kong say Edward Snowden, 30, has sought legal representation from human rights lawyers as he prepares to fight U.S. attempts to force him home for trial.

U.S. authorities have charged Snowden with theft of U.S. government property, unauthorized communication of national defence information and wilful communication of classified communications intelligence to an unauthorized person, with the latter two charges coming under the U.S. Espionage Act.

The United States and Hong Kong signed an extradition treaty which came into effect in 1998, a year after Hong Kong returned from British to Chinese rule. Scores of Americans have been sent back home for trial since then.

While espionage and theft of state secrets are not cited specifically in the treaty, equivalent charges could be pressed against Snowden under Hong Kong's Official Secrets Ordinance, legal experts said.

If Hong Kong authorities did not charge Snowden with an equivalent crime, authorities could not extradite him, lawyers said. In the absence of charges, Snowden was also theoretically free to leave the city, one legal expert said.

Simon Young, a law professor at the University of Hong Kong, said that while the first charge involving theft might readily find equivalence in Hong Kong, the latter two spying offences will likely attract "litigation and dispute" in the courts.

The timeframe for such proceedings remains unclear, but Hectar Pun, a barrister with human rights expertise, was quoted as saying such an extradition could take three to five years.

Under Hong Kong's extradition mechanism, a request first goes through diplomatic channels to Hong Kong's leader, who decides whether to issue an "authority to proceed". If granted, a magistrate issues a formal warrant for the arrest of Snowden.

Once brought before the court, the judge would decide whether there was sufficient evidence to commit Snowden to trial or dismiss the case, though any decision could be appealed in a higher court.

Snowden could claim political asylum in Hong Kong, arguing he would face torture back home. Article six of the treaty states extradition should be refused for "an offence of a political character".

"The unfairness of his trial at home and his likely treatment in custody" were important factors to consider for Snowden, said Young, the law professor, on Snowden's chances of claiming political immunity from extradition.

Should a Hong Kong court eventually call for Snowden's extradition, Hong Kong's leader and China could, however, still veto the decision on national security or defence grounds.

Snowden has admitted leaking secrets about classified U.S. surveillance programmes, which he said he did in the public interest. Supporters say he is a whistleblower, while critics call him a criminal and perhaps even a traitor. (Editing by Nick Macfie)