Ms Somerville, 48, and her architect boyfriend Jeremy Dixon, 56, were questioned by Scotland Yard's child-pornography unit on Thursday night, after Mr Dixon was met by detectives at a London chemist. He had gone to pick up what the couple insist were "family photographs".
Both Ms Somerville, who regained a top position in news broadcasting after a brain tumour operation, and Mr Dixon, who designed the extension to the Royal Opera House, strenuously denied the allegations. They said the pictures, which reportedly include a picture of Ms Somerville's daughter naked in the bath, were innocent. Ms Somerville also said she was "deeply distressed" that the arrests were made public, leading Scotland Yard to deny last night that it had leaked the news to the media.
Leading child-law experts said the case would renew the debate about the law surrounding child pornography. This has intensified over the decade, focusing in particular on whether society has become too sensitive to the possibility of child abuse - sometimes with traumatic results for innocent parties.
The most recent amendment to the law, outlawing the possession of pornographic photographs of children, was introduced seven years ago, amid intense lobbying from campaigners who included Mary Whitehouse. Although John Patten, then a Home Office minister, emphasised it was not the intention to catch innocent family snaps of naked children in the bath or on the beach, it was inevitable parents would come under more scrutiny.
"Clearly there is a risk innocent parents may be drawn in, and there is also a grey area here on what indecency means," said Alan Levy, QC, who specialises in child law. "This tests the law as to what we consider indecent. What it must really rely on is common sense. If you are taking naked pictures of a seven-year-old, which seems older than usual, you are playing with fire - not because you are necessarily doing anything indecent, but because it might be construed that way."
The pictures of Ms Somerville's daughter were reported to police by a member of staff at Boots. The chemist chain has a policy of reporting any photographs that staff believe may be obscene or pornographic.
The possession of child pornography carries a fine rather than a prison sentence, because according to the architects of the law the "shame of conviction... could be a greater punishment." However, under the 1978 Protection of Children Act it is also a crime to take pornographic pictures of children, or to permit them to be taken.
Ms Somerville and Mr Dixon were released early on Friday morning on police bail, pending further inquiries. Scotland Yard said that police had not removed anything yesterday during a search of the couple's north London home.
Campaigners who represent parents who have been wrongfully accused of child abuse said the system was letting parents and children down. "The problem is there are now so many investigations into various forms of child abuse and neglect that the system is being overloaded," said Sue Amphlett, director of Parents Against Injustice.
"Either children who are being abused are not being protected properly, or a whole new group of children who were never abused in the first place are being traumatised. Six out of seven of these investigations are dropped.
"The case depends on what these pictures are, but I would expect that in every family album there are photographs of children running round naked and getting out of the bath, and it is a question of interpretation."