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 British Conservative Party lawmaker, who said the justification appears to be without foundation and threatens 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 U.S. National Security Agency. Miranda was stopped at London's Heathrow airport in August and held by the Metropolitan Police for nine hours while on his way home to Brazil.