A deal to sell The Observer would reshape Britain's declining Sunday newspaper market. It will also raise questions about whether The Guardian plans to extend its six-day-a-week print operation to Sundays.
One source said that GMG had been given approval by The Scott Trust, which oversees The Guardian's stewardship, to explore a deal.
They added that there was a belief among GMG executives that the investment required to provide a robust digital future for The Observer was more likely to be provided by a third party as The Guardian focuses on its own internationally renowned reporting.
Mr Harding is a former editor of The Times and head of news at the BBC, and set up Tortoise Media in a bid to provide deeper insight into and analysis of significant global stories.
"We think the Observer is one of the greatest names in news," Mr Harding said.
"We believe passionately in its future - both in print and digital.
"We will honour the values and standards set under the Guardian's great stewardship and uphold the Observer's uncompromising commitment to editorial independence, evidence-based reporting and journalistic integrity.
"Like its many, many loyal readers, we admire the strength and heart of the Observer's reporting, we prize its original, unbiddable thinking and we love it for its passions: food, music, film and art.
"George Orwell described the Observer as 'the enemy of nonsense'; we're excited to show readers, old and new, that it still is."
The Observer is thought to employ fewer than 100 full-time and freelance staff on a standalone basis.
https://news.sky.com/story/the-observers-owner-confirms-sale-talks-with-tortoise-media-13216644