The flow of undocumented immigrants into the United States slowed down over the past year, according to a study released by the Pew Hispanic Centre on Thursday.

The research shows a fall in the total number of illegal immigrants, to 11.9 million in 2008 from 12.4 million in 2007. However, the centre would not formally say the number had in fact declined because of the study's margin of error.

"We cannot say with confidence that there was a decrease, but with full certainty there was a halt," said senior demographer Jeffrey Passel.

The study further confirmed that the flow of immigrants has been slowing since 2005. In the period from 2000-2004 the annual inflow of illegal migrants was about 800,000, while from 2005-2008 it was 500,000.

The research centre said it had no data as to why potential migrants declined to travel north or why those who had been in the United States chose to leave.

However, based on other studies, D'Vera Cohn - who co-wrote the Pew Centre report - suggested that "the slowdown of the economy" and the increase in police pressure on migrants were among the likely reasons.

Among an estimated 11.9 million illegal immigrants in the United States, 59 per cent come from Mexico, according to the Pew Centre, while a further 22 per cent are from the rest of Latin America and the Caribbean.

According to different studies by the centre, 63 per cent of Latinos in the United States claim that their situation worsened over recent years, 57 per cent say they fear their own deportation or that of a relative, a year later only 53 per cent said they were worried about it.