No 10 was “kept abreast” of the decision to detain David Miranda, the partner of Guardian journalist Glenn Greenwald, a spokesman has said.
Mr Miranda was held at Heathrow for nine hours on Sunday, while in transit from Germany to Brazil.
He has launched a legal challenge over the police’s use of anti-terror laws to detain him and seize his property.
But Home Secretary Theresa May said the police must act if someone had “highly sensitive stolen information”.
Mr Miranda, a 28-year-old Brazilian national, was held at Heathrow on his way from Berlin to Rio de Janeiro where he lives with Mr Greenwald. The Guardian said he had been carrying “journalistic materials” but was not an employee of the newspaper.