The detention of the partner of a former Guardian journalist has triggered fresh concerns after it emerged that a key reason cited by police for holding him under terrorism powers was the belief that he was promoting a "political or ideological cause".

The revelation has alarmed leading human rights groups and a Tory MP, who said the justification appeared to be without foundation and threatened to have damaging consequences for investigative journalism.

David Miranda is the partner of Glenn Greenwald, the journalist who – often in collaboration with the Guardian – has broken many stories about the extent and scope of spying by the US National Security Agency. Miranda was stopped at Heathrow airport in August and held by the Metropolitan police for nine hours while on his way home to Brazil.

Miranda, it has been claimed, was carrying some 58,000 encrypted UK intelligence documents. He had spent a week in Berlin visiting a journalist, Laura Poitras, who has worked with Greenwald on many of his stories, which have been based on information leaked by the former NSA contractor Edward Snowden.

Now documents referred to in court last week before a judicial review of Miranda's detention shine new light on the Metropolitan police's explanation for invoking terrorism powers – a decision critics have called draconian.

It became apparent during the court hearing that there were several drafts of the Port Circular Notice – the document used to request Miranda's detention under schedule 7 to the 2000 Terrorism Act – before the final version was submitted.

The draft that was finally used states: "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. We therefore wish to establish the nature of Miranda's activity, assess the risk that Miranda poses to national security and mitigate as appropriate."

The notice then went on to explain why police officers believed that the terrorism act was appropriate.

"We assess that Miranda is knowingly carrying material, the release of which would endanger people's lives. 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 and as such we request that the subject is examined under schedule 7."

Shami Chakrabarti, director of Liberty, said the police assessment represented a "chilling" threat to democracy. "More and more we are shocked but not surprised," she said. "Breathtakingly broad anti-terror powers passed under the last government continue to be abused under the coalition that once trumpeted civil liberties.

"The express admission that politics motivated the detention of David Miranda should shame police and legislators alike. It's not just the schedule 7 detention power that needs urgent overhaul, but a definition of terrorism that should chill the blood of any democrat."

Padraig Reidy of Index on Censorship, which campaigns for free speech, said that the police's justification for Miranda's detention was "very dangerous" for investigative journalism. "The whole point of such journalism is to find stuff the government doesn't want raised," he said. "The message this gives off is 'don't find this sort of stuff, or you will be treated as a terrorist'."

Greenwald was equally scathing, tweeting: "UK govt beats its mighty chest, now explicitly equates journalism with 'terrorism' and 'espionage'."

The home secretary, Theresa May, has criticised the Guardian's decision to publish the Snowden leaks. May has said she agrees with the assessment of Andrew Parker, the head of MI5, that the newspaper had damaged Britain's national security. But Conservative MP Dominic Raab said: "The assertion that national security has been undermined has been bandied around wildly and not explained in any cogent way."

And he questioned the police's handling of the Miranda affair. "If he was behaving in such a nefarious way why wasn't he arrested, charged and bailed?" Raab said. "If he was guilty of putting national security at risk, then why did they let him go?"

Gwendolen Morgan of Bindmans, Miranda's solicitors, said this week's judicial review will focus on whether the use of schedule 7 was disproportionate and whether it was incompatible with the inalienable right to freedom of expression.

"We will argue that draconian counter-terrorism powers were used in our client's case for an improper purpose," Morgan said. "Not to determine whether our client could in any sense be considered a 'terrorist', but rather to retrieve potentially embarrassing journalistic material in his possession."

The impact of Snowden's leaks on national security is expected to be addressed this week when parliament's intelligence and security committee will question the heads of MI6, MI5 and GCHQ in public for the first time.