The friend and retired BBC worker said the late Crimewatch host was told DJs, celebrities and other staff were involved in organised abuse.
But the anonymous source says “no one wanted to know” when Miss Dando raised concerns about the alleged ring and other sexual abuse claims at the BBC.
She is said to have passed a file to senior management in the mid-1990s, but they never carried out an investigation.
The 37-year-old TV presenter was shot dead on April 26, 1999, on the doorstep of her home in west London. The crime remains unsolved.
“I don’t recall the names of all the stars now and don’t want to implicate anyone, but Jill said they were surprisingly big names.
“I think she was quite shocked when told about images of children and that information on how to join this horrible paedophile ring was freely available.
“Jill said others had complained to her about sexual matters and that some female workmates also claimed they had been groped or assaulted. Nothing had been done and there seemed to be a policy of turning a blind eye.”
The former colleague said female BBC staff confided in Jill, one of the best-known TV faces of the day after fronting primetime shows including Holiday and the Six O’Clock News as well as Crimewatch.
The source said:
“I think it was in the mid-1990s. She was seen as the face of the BBC and a magnet for women with problems.”
Miss Dando compiled a file of complaints but was a television presenter and “not really an investigative journalist”, the source said.
“She passed the information on to someone else and they gave it back. No one wanted to know. I do remember that she gave a file to senior management. I don’t think she heard any more.
“Other women who complained told Jill they didn’t want to risk their careers by making official statements against individuals as they would lose their jobs and that bosses seemed to ignore it.”
A BBC spokesman said they would always investigate any allegations of this nature, but added:
“We’ve not seen anything that substantiates these claims.”
After Jimmy Savile’s sexual crimes were exposed in 2012, Jill’s fellow BBC presenter Sally Jones revealed that the disgraced DJ tried to kiss and grope her in a lift in the late 1980s.
At the time, she told Jill, who said Savile was “just a dirty old perv”.
The Crimewatch host “had to fend off plenty of unwelcome advances herself”, said Sally.
DJ Liz Kershaw and former Countryfile presenter Miriam O’Reilly have also both claimed they suffered sexual harassment at the BBC in the 1980s.
Miss Dando joined a campaign to help children spot paedophiles the year before she died.
She had received death threats, putting the BBC in lock-down, with armed guards patrolling Television Centre in London.
Her death sparked one of Britain’s biggest murder hunts. Barry George was found guilty in 2001, but his conviction was quashed in 2008.
Theories about her death range from the culprit being a Serbian warlord angry over her TV appeal on behalf of Kosovan refugees, to British underworld enforcers furious at her Crimewatch investigations.
Miss Dando was also looking into the death of her friend, Chelsea vice-chairman Matthew Harding, who was killed in a helicopter crash.
By John Carr