Carolyn McCall has been appointed chief executive of Guardian Media Group, succeeding Sir Robert Phillis, who retires at the end of July.
Ms McCall, currently chief executive of GMG's national newspaper division, Guardian Newspapers Limited, will take up her new post on August 1.
She will also replace Sir Robert on the 10-strong board of the Scott Trust, which was set up in 1936 to secure the financial and editorial independence of the Guardian and is the owner of GMG.
Sir Robert announced in January that he planned to retire as GMG chief executive on health grounds. He will remain on the GMG board as a non-executive director.
"Carolyn has a brilliant track record in running our national newspapers, and has a very clear and compelling vision for the future growth and strategy of the group," said Paul Myners, the chairman of GMG.
In her new role, Ms McCall will oversee a business that encompasses the Guardian and Observer national newspapers, the Guardian Unlimited website operation including MediaGuardian.co.uk, the Manchester Evening News and a portfolio of other regional newspapers in the north-west, Surrey and Berkshire.
GMG also owns five radio stations under the Real Radio and Smooth FM brands and Trader Media Group, which publishes motoring classified magazine Auto Trader and its website, as well as nearly 70 publications in Britain, Ireland, the Netherlands, Italy and South Africa.
GMG announced on Friday that it planned to float a minority stake in Trader Media.
Ms McCall has held senior management positions at GMG for more than a decade and is also a non-executive director of Tesco and chair of Opportunity Now, the UK's leading membership organisation for gender equality and diversity at work.
She became advertising director of Guardian Newspapers in 1995, commercial director two years later and deputy managing director in 1998.
Ms McCall was appointed to her current job in 2000, when she also joined the GMG board.
She is also a former non-executive director of New Look and a former president of Women in Advertising and Communications London.
Sir Robert said: "This is a first-class appointment. Carolyn's drive, energy and understanding will ensure continuing success for GMG in the digital age. She has my total support."
Ms McCall added: "With its powerful media brands and strong financial position, GMG is well placed to take advantage of all the digital and other opportunities that are being created by the enormous changes in the media market."
Liz Forgan, the chair of the Scott Trust, said: "Carolyn McCall has an exceptional understanding of the purposes and values of the trust through her long experience at GNL. She will be a great asset to our work, and we are delighted to have her working with us."
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