More than three million people have been displaced by the conflict in Iraq since the start of 2014, the UN says.
A statement by the International Organization for Migration (IOM) said two-thirds were from the provinces of Anbar, Nineveh and Salahuddin.
The regions have been hardest hit by fighting between Islamic State (IS) militants and pro-government forces.
More than 276,000 people were displaced over the past two months amid fighting over Anbar’s capital, Ramadi.
The city fell to IS in mid-May after the Iraqi army withdrew.
Since then, pro-government forces led by Shia militias have launched a major operation to regain Ramadi and drive the jihadist group out of Anbar.
The IOM said on Tuesday that at least 3.09 million Iraqis had fled their homes since January 2014, when IS militants overran parts of Ramadi and took control of the nearby city of Falluja, only 70km (45 miles) from the capital Baghdad.
Six months later, half a million Iraqis, many of them members of ethnic or religious minorities, fled their homes to escape an IS offensive that saw the group capture the northern city of Mosul before sweeping southwards towards Baghdad and declaring the creation of a “caliphate”.
Earlier this month, the UN’s Humanitarian Co-ordinator for Iraq, Lise Grande, said more than eight million people in the country required immediate life-saving support, a number that could reach 10 million by the end of 2015.
Ms Grande made an urgent appeal for $497m (£316m) to cover the cost of providing shelter, food, water and other assistance.
“The crisis in Iraq is one of the most complex and volatile anywhere in the world,” she said, warning that more than 50% of the UN’s aid operations would have to be shut down or cut back if funding was not received immediately.