London, Jun 18 (IANS): Some 860,000 immigrants may be illegally living in the UK, the National Audit Office (NAO) said in an official report

"Its (the UK Home Office's) last estimate was around 430,000 people in 2005. More recent estimates from other organisations suggest the population might have doubled, although the NAO has not validated these estimates," Xinhua news agency quoted the report released on Wednesday as saying.

The Home Office encourages people to return voluntarily, but those returns have reduced, from an average of 1,200 a month in 2015 to approximately 460 in 2019, said the independent public spending watchdog.

It has enforced the removal of 7,400 of the 13,100 immigration offenders returned in the 12 months to the end of November 2019.

Almost 5,000 of those were foreign national offenders, the NAO report said.

The report said the Home Office is helping to stop more people from entering Britain without proper documentation or through clandestine means.

According to the NAO, the Home Office's vision for Immigration Enforcement is to "reduce the size of the illegal population and the harm it causes".

It supports this vision through its operational missions of preventing illegal migration through greater compliance with laws, tackling threats associated with immigration offending and maximizing returns of the illegal population and foreign national offenders to their country of origin.