McDonald's has been trying to shed its unhealthy reputation

He had been diagnosed with the disease shortly after taking over from Jim Cantalupo, who died suddenly of a heart attack in April.

Jim Skinner, a veteran of 33 years at the company, will assume the top job.

McDonald's has ridden out a rough patch in recent years, switching to healthier options and seeing profits surge.

Among the firm's innovations are a greater range of salads and fruit, and in some cases smaller portions.

The three months to September had seen a 42% rise in earnings.

Smooth takeover

Mr Bell's need for therapy for colorectal cancer has disrupted his work, including a gathering of managers from around the world which took place in Australia earlier this month.

He will remain chairman of the board, the company said.

The shake-up at the top was unlikely to change the company's path, analysts said, since all those concerned were long-term insiders.

Mr Bell, now 44, started work in a McDonald's outlet in his native Sydney when he was 15.

By 19, he was the youngest store manager in the country, and has been with the group ever since.

The new chief executive, Jim Skinner, started at the group in 1971 as a trainee manager, and has since served in positions with oversight of almost every market in which the firm operates.