* David Leung is 25-year veteran of prosecutions department
* Says he was sidelined from cases under new security legislation
* Leung is first senior HK official to quit since new laws
* Says every defendant should have "fair trial" (Adds response from justice secretary and context)
By James Pomfret
HONG KONG, July 31 (Reuters) - Hong Kong's head of public prosecutions quit this week, citing differences with the city's top legal official, and after being sidelined from cases under new China-imposed national security legislation, according to an email obtained by Reuters.
The resignation of David Leung is the first involving a senior Hong Kong official since Beijing imposed the harsh security laws on the city, and is a sign of discomfort within the government regarding the new arrangements.
Leung, the city's director of public prosecutions, who recently led a high-profile public prosecution against pro-democracy leaders involved with the city's 2014 Occupy Central protests, wrote in the email that he could no longer work with Hong Kong's justice secretary Teresa Cheng.
"It is most unfortunate that I do not see eye to eye with the SJ (Secretary for Justice) on the running of PD (Prosecutions Division), and the situation has not improved with the passage of time," he wrote in the email obtained by Reuters.
Cheng's spokesman confirmed in a statement that Leung had submitted his resignation.
Leung noted in his email concerns and uncertainty within the department over national security cases that have arisen since the new laws took effect on June 30 but said he had no knowledge of proceedings.
"As to National Security Cases, I do not have information for you as my assistance and participation in those matters have not been required," he wrote.
Leading Hong Kong lawyers have already warned of a stark new era of mainland justice. Hong Kong's proudly independent judiciary, one of many freedoms guaranteed when the former British colony returned to Chinese rule 23 years ago, has long been considered key to its success as a global financial hub.
A source with direct knowledge of the matter said it was "shocking" that the head of Hong Kong's prosecutions division could be sidelined in this manner.
"He's been completely cut out from national security matters," said the source, who declined to be named given the sensitivity of the matter.
"Yet these are prosecutions in Hong Kong, and he's the head of public prosecutions. It undermines the rule of law ... and established practices," the source added.
Hong Kong's Justice Secretary said in the statement that she had always worked in "an objective and professional manner and respected the opinions of colleagues". She added it was inappropriate that the internal email was made public.
The new security legislation calls for the setting up of a specialised prosecution division to deal with offences endangering national security, with prosecutors of this division to be appointed by the justice secretary.
Leung, a 25-year veteran of the prosecutions division, said in his email that every defendant should have a "fair trial" based on available evidence.
"Being a prosecutor is a difficult but honourable job," wrote Leung, who said he plans to leave his post at year-end. (Reporting By James Pomfret, Editing by Anne Marie Roantree and Gareth Jones)