Earl Spencer said it was "beyond belief" the BBC rehired Martin Bashir as Religious Affairs Correspondent, describing him as a "serial liar".
A report into why the journalist was rehired in 2016 after using forged documents is due to be published next week.
An inquiry by Lord Dyson concluded the broadcaster covered up "deceitful behaviour" by Bashir to get the interview with Princess Diana at Kensington Palace in 1995.
Speaking to ITV News London at the launch of a new book, Earl Spencer said he was eagerly waiting to find out what the report says.
"It does seem so odd really because they knew he was a serial liar and the irony of giving him the religious post is beyond belief really," said Earl Spencer.
"I gather the report is out any day, it will be interesting to see how on earth they re-employed him especially after he had a very chequered career in between.
"I think it's important to find out why they reemployed him. Is it for a dark reason that some people suspect? That maybe they had to on some basis - there seems no logical reason for doing so, it'll be interesting to find out," he added.
Martin Bashir has previously said Princess Diana was pleased with the interview at the time.
The BBC said the "matter was being looked into and we will publish shortly".
Earl Spencer also spoke about the arrival of his great niece after Harry and Meghan announced the birth of their second child, Lilibet “Lili” Diana Mountbatten-Windsor.
"It's fantastic - not a surprise! I thought Diana would be in there and how lovely, it's such a distinctive name and of course it comes with so much emotion for them - wonderful that she is remembered by another generation," he said.
The Earl said he found out about the new royal arrival in an email from Harry and he hoped he would be able to go to America soon to see them.
Earl Spencer spoke at the launch of his new book The White Ship: Conquest, Anarchy and the Wrecking of Henry I's Dream, the story of one of the greatest maritime disasters England has ever seen which altered the course of English and European history.
"The White Ship was the Titanic of its day went down and there were 300 very important people killed in the wreck but the most important was the heir to the throne," Earl Spencer said.
"He was killed and there wasn't a replacement so it ended in anarchy! Chronicalists at the time said blood descended on the land so it was an astonishing disaster," he added.
For more information about The White Ship: Conquest, Anarchy and the Wrecking of Henry I’s Dream - click here.