The number of illegal immigrants living in the US hit its lowest level in a decade under President Barack Obama's administration, a study suggests.

There were 11 million undocumented immigrants living in the US in 2015 according to the Pew Research Centre, the lowest number since 2005.

That included people who crossed the border illegally and those who overstayed their legal visas, and was based on data from the Census Bureau's American Community Survey.

The Centre also released a preliminary estimate of 11.3 million illegal immigrants in 2016, based on a different data source with a smaller sample size.

Trump doesn't say who will pay for Mexico wall, only that it's coming

Donald Trump focused on the constant flow of migrants from Mexico from the beginning of his campaign, denouncing border crossers as "criminals" and "racists".

Mexicans have long been the largest group among undocumented immigrants, the Centre noted, but their numbers have declined since peaking at 6.9 million in 2007.

In 2015, the Centre's estimate said the number declined to 5.6, making up 51 per cent of the total undocumented immigrant population.

The preliminary estimates are based on census data from March 2016, in the last year of Mr Obama's second term, meaning they do not account for any impact of Mr Trump's policy changes.

The controversial orders Donald Trump has already issued Show all 9 1 /9 The controversial orders Donald Trump has already issued The controversial orders Donald Trump has already issued Trump and the media White House Press Secretary Sean Spicer takes questions during the daily press briefing Getty Images The controversial orders Donald Trump has already issued Trump and the Trans-Pacific Partnership Union leaders applaud US President Donald Trump for signing an executive order withdrawing the US from the Trans-Pacific Partnership negotiations during a meeting in the Roosevelt Room of the White House in Washington DC. Mr Trump issued a presidential memorandum in January announcing that the US would withdraw from the trade deal Getty The controversial orders Donald Trump has already issued Trump and the Mexico wall A US Border Patrol vehicle sits waiting for illegal immigrants at a fence opening near the US-Mexico border near McAllen, Texas. The number of incoming immigrants has surged ahead of the upcoming Presidential inauguration of Donald Trump, who has pledged to build a wall along the US-Mexico border. A signature campaign promise, Mr Trump outlined his intention to build a border wall on the US-Mexico border days after taking office Getty Images The controversial orders Donald Trump has already issued Trump and abortion US President Donald Trump signs an executive order as Chief of Staff Reince Priebus looks on in the Oval Office of the White House. Mr Trump reinstated a ban on American financial aide being granted to non-governmental organizations that provide abortion counseling, provide abortion referrals, or advocate for abortion access outside of the United States Getty Images The controversial orders Donald Trump has already issued Trump and the Dakota Access pipeline Opponents of the Keystone XL and Dakota Access pipelines hold a rally as they protest US President Donald Trump's executive orders advancing their construction, at Columbus Circle in New York. US President Donald Trump signed executive orders reviving the construction of two controversial oil pipelines, but said the projects would be subject to renegotiation Getty Images The controversial orders Donald Trump has already issued Trump and 'Obamacare' Nancy Pelosi who is the minority leader of the House of Representatives speaks beside House Democrats at an event to protect the Affordable Care Act in Los Angeles, California. US President Donald Trump's effort to make good on his campaign promise to repeal and replace the healthcare law failed when Republicans failed to get enough votes. Mr Trump has promised to revisit the matter Getty Images The controversial orders Donald Trump has already issued Donald Trump and 'sanctuary cities' US President Donald Trump signed an executive order in January threatening to pull funding for so-called "sanctuary cities" if they do not comply with federal immigration law AP The controversial orders Donald Trump has already issued Trump and the travel ban US President Donald Trump has attempted twice to restrict travel into the United States from several predominantly Muslim countries. The first attempt, in February, was met with swift opposition from protesters who flocked to airports around the country. That travel ban was later blocked by the Ninth Circuit Court of Appeals. The second ban was blocked by a federal judge a day before it was scheduled to be implemented in mid-March SANDY HUFFAKER/AFP/Getty Images The controversial orders Donald Trump has already issued Trump and climate change US President Donald Trump sought to dismantle several of his predecessor's actions on climate change in March. His order instructed the Environmental Protection Agency to reevaluate the Clean Power Plan, which would cap power plant emissions Shannon Stapleton/Reuters

A huge drop in the number of people arrested crossing the Mexico border into the US has been recorded since Mr Trump took office, with his administration seeking to take credit for the decline.

Fewer than 12,500 were caught at the southern border in March, the lowest monthly figure in at least 17 years and the second month in a row border arrests dropped sharply.

The White House issued a statement to say Mr Trump's "commitment to securing our border and supporting law enforcement is already showing results."

Loading....

However, those working in shelters and experts on migration said it will take several more months to judge whether any drop-off is long lasting, and that the numbers could surge again as quickly as they have fallen.

Some thought the real "Trump effect" pushed fearful people to move up their journeys and get to the US before Mr Trump took office.