The partner of a journalist with links to Edward Snowden, the fugitive former U.S. spy agency contractor, was detained at London's Heathrow Airport on August 18.
Reports said David Miranda was held for nearly nine hours under antiterror legislation.
Miranda is the partner of Glenn Greenwald, who has reported for Britain's "Guardian" newspaper on U.S. global-surveillance programs based on leaks provided by Snowden.
Posting on "The Guardian" website, Greenwald said Miranda's cell phone, laptops, and memory sticks were confiscated.
A statement from "The Guardian" said it was "dismayed" at Miranda's detention and that it would be pressing British authorities for an urgent clarification.
The Brazilian government expressed "grave concern" over the detention of Miranda, Greenwald's longtime partner with whom he's in a civil union.
The pair live in Rio de Janeiro.
Based on reporting by AP and Reuters