By Mark Hosenball

WASHINGTON, Nov 1 (Reuters) - British authorities claimed the domestic partner of reporter Glenn Greenwald was involved in "terrorism" when he tried to carry documents from former U.S. intelligence contractor Edward Snowden through a London airport in August, according to police and intelligence documents.

Greenwald's partner, David Miranda, was detained and questioned for nine hours by British authorities at Heathrow on Aug. 18, when he landed there from Berlin to change planes for a flight to Rio De Janeiro, Brazil.

After his release and return to Rio, Miranda filed a legal action against the British government, seeking the return of materials seized from him by British authorities and a judicial review of the legality of his detention.

At a London court hearing this week for Miranda's lawsuit, a document called a "Ports Circulation Sheet" was read into the record. It was prepared by Scotland Yard - in consultation with the MI5 counterintelligence agency - and circulated to British border posts before Miranda's arrival. The precise date of the document is unclear.

"Intelligence indicates that Miranda is likely to be involved in espionage activity which has the potential to act against the interests of UK national security," according to the document.

"We assess that Miranda is knowingly carrying material the release of which would endanger people's lives," the document continued. "Additionally the disclosure, or threat of disclosure, is designed to influence a government and is made for the purpose of promoting a political or ideological cause. This therefore falls within the definition of terrorism..."

Miranda was not charged with any offense, although British authorities said in August they had opened a criminal investigation after initially examining materials they seized from him. They did not spell out the probe's objectives.

A key hearing on Miranda's legal challenge is scheduled for next week. The new details of how and why British authorities decided to act against him, including extracts from police and MI5 documents, were made public during a preparatory hearing earlier this week.

British authorities have said in court that items seized from Miranda included electronic media containing 58,000 documents from the U.S. National Security Agency and its British counterpart, Government Communications Headquarters (GCHQ).

Greenwald, who previously worked for Britain's Guardian newspaper, has acknowledged that Miranda was carrying material supplied by Snowden when he was detained.

In an email to Reuters, Greenwald condemned the British government for labeling his partner's actions "terrorism."

"For all the lecturing it doles out to the world about press freedoms, the UK offers virtually none...They are absolutely and explicitly equating terrorism with journalism," he said.

Separately on Friday, media disclosed details of an open letter Snowden issued to Germany from his place of exile in Russia, in which he says his revelations have helped to "address formerly concealed abuses of the public trust" and added that "speaking the truth is not a crime."

Snowden said he was counting on international support to stop Washington's "persecution" of him for revealing the scale of its worldwide phone and Internet surveillance.

Steven Aftergood, a secrecy expert with the Federation of American Scientists, said that given the nature of the material that Miranda was carrying, a harsh response by British authorities was not unexpected.

"It seems that UK authorities were attempting to seize or recover official documents, to which they arguably have a claim," Aftergood said. "The authorities' action was harsh, but not incomprehensible or obviously contrary to law."

In a separate document read into the court record, MI5, also known as the Security Service, indicated British authorities' interest in Miranda was spurred by his apparent role as a courier ferrying material from Laura Poitras, a Berlin-based filmmaker, to Greenwald, who lives with Miranda in Brazil.

"We strongly assess that Miranda is carrying items which will assist in Greenwald releasing more of the NSA and GCHQ material we judge to be in Greenwald's possession," said the document, described as a "National Security Justification" prepared for police.

"Our main objectives against David Miranda are to understand the nature of any material he is carrying, mitigate the risks to national security that this material poses," the document added.

A spokesman for the British Embassy in Washington had no comment on the court proceedings or documents.