WASHINGTON -- The world has more poor people than previously thought, the World Bank said in a new study Tuesday, adding that progress to reduce poverty is being made in all regions except Africa.

In a major revision of the method used to calculate poverty, the World Bank said that 1.4 billion people in the poorest developing countries lived below $1.25 a day in 2005, representing one in four people.

That compared with 1.9 billion in 1981, or one in two people, the Bank said.

The researchers calculated their estimates using a new poverty line of $1.25 a day for 2005 in the 10-20 poorest countries, based on improved internationally comparable price data. The previous figure was $1 a day.

"As a consequence of the improvements in price data collection and processing, we have discovered that the cost of living is higher in the developing world than we thought," the Bank said in a statement.

Without this cost-of-living revision, the number of poor would have fallen below one billion.

"The new data confirm that the world will likely reach the first Millennium Development Goal of halving the 1990 level of poverty by 2015 and that poverty has fallen by about one percentage point a year since 1981," said Justin Lin, the Bank's chief economist.

"However, the sobering news that poverty is more pervasive than we thought means we must redouble our efforts, especially in Sub-Saharan Africa," he said.

The number of poor has fallen by 500 million since 1981 -- from 52 percent of the developing world's population in 1981 to 26 percent in 2005, the 185-nation development lender said.

Sub-Saharan Africa was the only major region where the fight against poverty stalled, the data showed.

In 1981, 51 percent of the people in Sub-Saharan Africa lived below the poverty line. That number spiked to 58 percent in 1996 and fell back to 50 percent in 2005. The number of poor almost doubled in the 1981-2005 period, from 200 million to 380 million.

The World Bank highlighted a sharp reduction in poverty in East Asia, the poorest region in the world in 1981, when 80 percent of the people lived below the $1.25 line. By 2005, that number had fallen to 18 percent.