ROME (AP) — The United Nations said Tuesday its 2009 headline-grabbing announcement that 1 billion people in the world were hungry was off-target and that the number is actually more like 870 million.

The U.N. Food and Agriculture Organization blamed flawed methodology and poor data for the bum projection, and said it now uses a much more accurate set of parameters and statistics to calculate its annual estimate of the world’s hungry.

FAO issued its 2012 state of food insecurity report on Tuesday, and its core point was to set the record straight about the number of the world’s undernourished people, applying the more accurate data retroactively to 1990.

And the good news, FAO said, is that the number of hungry people has actually been declining steadily — rather than increasing — over the past two decades, although progress has slowed since the 2007-2008 food crises and the global economic downturn.

‘‘We have good news, we have made some progress in reducing hunger,’’ Jose Graziano da Silva, the FAO director-general, told a press conference launching the report.

FAO said that if the right action is taken now to boost economic growth and invest in agriculture, particularly in poor countries, the U.N. goal of reducing by one-half the number of the world’s hungry people by 2015 is very much within reach.

To be sure, 870 million hungry people is still far too many hungry people, said the heads of the three U.N. food agencies in a forward to the report.

‘‘In today’s world of unprecedented technical and economic opportunities, we find it entirely unacceptable that more than 100 million children under the age of five are underweight, and are therefore unable to realize their full socio-economic and human potential,’’ they wrote.

FAO made headlines in 2009 when it announced that 1 billion people — one-sixth of the world’s population — were undernourished. A high-level summit was called at FAO headquarters in Rome, where the pope spoke. The U.N. chief went on a daylong hunger strike to show solidarity with the 1 billion. The Group of Eight devoted much of its summit that year to pledging $20 billion for seeds, fertilizers and tools to help poor nations feed themselves.

It turns out, though, that the projections were wrong. They were calculated using figures from non-U.N. sources that were fed into the U.N.’s number-crunching model, because FAO was expected to quickly come up with an estimate of how many people might go hungry from the dual crises of high food prices and the global downturn, said Kostas Stamoulis, director of FAO’s agricultural development economics division.

‘‘There was considerable fear that that combination of lower incomes and higher prices was going to cause significant undernourishment,’’ said Jomo Kwame Sundaram, FAO’s assistant director-general for economic and social development.

But now, ‘‘no one really knows for sure if at any particular period whether that 1 billion figure was actually reached or not,’’ he said of the three year period. He said the goal of the state of food insecurity report is to assess chronic hunger over time, rather than spikes caused by temporary food shortages and price hikes.

What the U.N. couldn’t know at the time was how well governments would respond to the crises to protect their poorest, and how individual families would make sacrifices — such as in health care or education — to make sure they had enough food on the table, said Jomo.

‘‘So it was in a way a mistake, an error, that we have made with many other organizations at the time,’’ said Pietro Gennari, the statistics director for the FAO’s economic and social development department.

Already, in 2010 FAO said the number of the world’s hungry was down to 925 million, though it didn’t explain how it came to that figure. On Tuesday, FAO officials said the new number of hungry people for 2012 was 870 million, and the organization revised all of its figures from 1990 using a new methodology based on:

—New population data from the U.N. Population Fund. China’s population estimate for the 1990s, for example, has been revised upward by some 25 million people, whereas Bangladesh’s population has been revised downwards by 17 million people. That affects hunger statistics because food production figures are divided by population figures.