The Daily Mirror broke the story in February 2001, claiming that Campbell was attending the self-help group to help cure her drug addiction. The story was accompanied by pictures showing her leaving a meeting.
Campbell sued the newspaper, claiming that the report amounted to a breach of confidence and a violation of both the Human Rights Act and the Data Protection Act. The High Court accepted her arguments and ordered the Mirror to pay damages of £3,500. The newspaper appealed.
In October 2002, three Appeals Court judges overturned that decision, ordering her to repay the damages and to pay the Mirror's £350,000 costs. They reasoned that the report was in the public interest, because Campbell had publicly denied taking drugs.
Lord Phillips had ruled that, where a public figure chose to make untrue pronouncements about his or her private life, the press would normally be entitled to put the record straight.
The Appeals Court also found that the journalist had to be given reasonable latitude as to the manner in which the story was conveyed to the public or else his right to freedom of expression under the European Convention on Human Rights would be "unnecessarily inhibited."
Campbell's appeal to the House of Lords is neither arguing that the Mirror was wrong to report the fact that she had used drugs, nor to report that she was receiving treatment for her addiction. Instead, her complaint is that the newspaper breached her confidence and invaded her privacy when it published details of her treatment, including the frequency of her attendance at therapy sessions.
She argues that "therapy is essentially private" and that there had been a breach of confidence by an unknown source who tipped off the Mirror.
Campbell will attempt to convince the five Law Lords that the existing law of confidence can be used to protect privacy and that this can be balanced with the law on freedom of expression. The Mirror argues that there is no law of privacy in the UK.
The judgment is expected to take at least six weeks.
Campbell's appeal coincides with complaints to British tabloids from lawyers for Jonny Wilkinson over the publication on Tuesday of photographs showing the England rugby star relaxing on holiday with his girlfriend in the Seychelles.