The BBC is to publish the findings of an internal investigation on Thursday into how the journalist Martin Bashir secured an explosive 1995 interview with princess Diana.

Questions have long been asked about how Bashir persuaded Diana to talk, with the princess' own brother claiming he used faked documents and bank statements.

The interview, on the BBC's flagship "Panorama" programme in November 1995, was watched by a record 22.8 million people and lifted the lid on Diana's troubled marriage to Prince Charles.

She famously said "there were three people" in her marriage -- her, Charles and his long-time mistress and now wife, Camilla Parker-Bowles, and also admitted adultery.

Bashir, 58, was little-known at the time of the interview but went on to have a high-profile career on US television networks, interviewing big-name stars, including the singer Michael Jackson, before returning to work for the corporation as religion editor.

He stepped down last week, citing ill health, hours before the report was submitted to BBC director-general Tim Davie. A scheduled BBC programme into the controversy was also delayed.

British media said Thursday the six-month probe by a retired senior judge had concluded Bashir used underhand methods to convince Diana to talk, in breach of editorial guidelines.

The Daily Telegraph suggested the report's findings could be comparable to phone-hacking revelations at the News of the World tabloid, which forced its closure.

A previous BBC inquiry had cleared Bashir of wrong-doing.

Diana and Charles formally divorced in 1996. She died aged 36 in a high-speed car crash in Paris the following year. Charles married Camilla in 2005.