The owner of The Guardian has collected a £600m-plus windfall from selling its 50.1 per cent stake in the car classifieds brand Auto Trader to the private-equity firm Apax Partners.

Guardian Media Group's (GMG) long-rumoured sale of Auto Trader's parent company, Trader Media Group, should guarantee the future of the loss-making flagship newspaper over the next decade.

Those close to GMG said that the cash would transform its balance sheet and continues a pattern of selling assets including the Manchester Evening News and Smooth Radio. Apax bought 49.9 per cent of Trader for £675m in 2007. Today's sale is said to be in the range of £600m to £700m.

The Guardian and Observer newspapers slumped to an operating loss of £31m in the year to March 2013. However, that was an improvement on a year ago when they lost £44m. Overall, GMG swung back into the black with a pre-tax profit of £22.7m.

Its chairman Neil Berkett said: "This deal will make GMG a very well-capitalised media organisation with the financial flexibility to navigate the rapidly changing environment."