The owner of the Guardian has confirmed it is in talks to sell the Observer, the world's oldest Sunday newspaper, to Tortoise Media.

Tortoise has approached Guardian Media Group (GMG) with an offer to invest around £25m over the next five years on the "editorial and commercial renewal" of the Observer.

Tortoise was launched five years ago by James Harding, a former BBC News chief and a former editor of the Times newspaper.

The Guardian reported that the title will remain a seven-day-a-week digital operation regardless of the outcome of negotiations with Tortoise about the Observer.

Observer staff were told that the investment would "help to safeguard its future" as a standalone product.

GMG is not actively trying to sell the Observer, but it is examining the Tortoise proposal to see if it is viable.

Founded in 1791, the Observer is the world's oldest Sunday newspaper, with a staff of around 70.