The choice of the relatively low-profile Sakira Oramo comes as a surprise, as Mr Rattle's 21-year-old protege, Daniel Harding, was widely touted to succeed him. Sir Simon was an unknown 25-year-old when he was appointed in 1980.
Mr Oramo, 31, a professional conductor for three years, is co-principal conductor of the Finnish Radio Symphony Orchestra and his initial contract as principal conductor and artistic adviser will be for three years.
"It was an unexpected surprise and an honour to be invited to become principal conductor of the CBSO after only my second visit to them in July this year," Mr Oramo said yesterday. "I am looking forward to the challenge of shaping the future of this marvellous orchestra in my own way while maintaining the legendary good work of my predecessor."
Mr Oramo, who has conducted the City of Birmingham Symphony Orchestra twice before, will conduct at least 30 concerts a year.
His rivals at the top of a "very long list" were either unavailable or unsuitable, said Edward Smith, the orchestra's chief executive. "Sakari was on the list and when he came in July it was quite clear to everyone on the committee that this was the ideal man to take over."
One factor that apparently gave Mr Oramo an edge was the enthusiasm of the players after his two concerts with them, the first in May last year. "By the time he had conducted the orchestra on his second visit there was an extraordinary fusion of opinion that he was just the right person to succeed Simon," said Peter Thomas, the orchestra leader. "Everyone in the orchestra is very motivated to make a success of this splendid new relationship."
Mr Oramo lives in Germany, where his wife, the soprano Anu Komsi, is contracted to the Bremen Opera until 1998. The couple have a five-year-old son.
He made his professional conducting debut with the Finnish Radio Symphony Orchestra at a few hours' notice in January 1993, when the scheduled conductor was taken ill. Since then he has conducted all the major Scandinavian orchestras and has forthcoming concerts in Germany, Switzerland and Britain.
The charismatic Sir Simon has built a reputation at the CBSO by mixing "difficult" contemporary music with favourites such as Elgar. He announced in February that he would quit the role of music director when his contract expired in 1998.