Up to 181,000 migrants who should have left the UK over the past two years could still be in the country illegally, new figures have shown.
The UK Border Agency (UKBA) estimate shows it does not do enough work to check
migrants are leaving the country if they have no right to remain, the National Audit Office
said.
But immigration minister Damian Green said the report showed exactly why radical reforms to the UK's immigration system were needed, and added that changes were already being introduced.
Auditors found migrants and employers could exploit the points-based system for visas because the UKBA has failed to check the risks posed by the process.
They have called for the UKBA to prioritise ensuring that migrants leave when they should - and to set national targets to tackle those who overstay their visas.
The Audit Office also said the agency "lacks the ability to easily identify individuals whose visas have expired".
"The agency estimates there may be up to 181,000 migrants in the UK of all visa types whose permission to remain has expired since December 2008," the NAO added.
Auditors said the agency could not verify that employers had first checked whether any British workers were suitable for positions at the point a migrant applies for a visa.
The agency has "not assessed the overall risk that relying on supporting documentation poses to proper control".
Margaret Hodge, chairwoman of the committee of public accounts, said after the UKBA refused extensions to visas it could not "guarantee that these people actually leave the country".
"The agency relies on employers to police their own employees and does not carry out its own checks," she said.
"There could be as many as 181,000 people here on expired visas."
Ms Hodge went on: "Gaps in data, poor risk management and inefficient processes mean that we cannot be certain that (the points-based system) either ensures proper controls or meets the UK's need for skilled labour."
Mr Green said radical reforms were already being introduced, including the "introduction of an annual limit on economic migrants, sweeping changes to the student visa system and a shake-up of the family and settlement route".
He added the agency was also committed to reintroducing exit checks by 2015.
"Counting people in and out of the country will give us better control over those that overstay," he said.
"We are determined to reduce net migration to the tens of thousands, and clamp down on immigration abuses."
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