Thousands of illegal immigrants could be granted an amnesty to stay in the UK to clear a massive backlog of cases, MPs fear.
The number of outstanding immigration and asylum cases rose by 25,000 in three months and is "spiralling out of control".
The Border Agency (UKBA) caseload is now equivalent to the population of Iceland, said Keith Vaz, chairman of the Commons Home Affairs Committee.
More than 300,000 cases needed to be dealt with at the end of June, up 9% over the previous three months, a committee report indicated.
The Border Agency had 302,064 cases to investigate, trace or conclude by the end of June - more than 25,000 compared with the end of March, figures showed.
Most of the increase came from a rise of more than 24,000 missing migrants denied permission to stay in the UK but whose whereabouts are unknown.
The outstanding cases also included 95,000 in the so-called controlled archives.
These are effectively the backlog of immigration and asylum cases, which the UKBA has pledged to close by the end of the year.
But to do this it will have to assess all these cases in three months when only 29,000 were removed from the archive in a year, the report said.
"We are concerned that the closure of the controlled archives may result in a significant number of people being granted effective amnesty in the United Kingdom, irrespective of the merits of their case," the MPs said.
While many of the people in the backlog of cases will have already left the country, "we are not convinced that the agency's limited checking regime will have picked up all of the applicants who remain in the country", the MPs went on.
"For this reason we are concerned that the final checks made on these cases should be thorough and that they should not be rushed to meet an artificial deadline."
The report also questioned the consequences for both the individual and the taxpayer if applicants whose cases are closed are then found in the UK.
Mr Vaz added: "There are now about the same number of cases awaiting resolution by UKBA as there are people living in Iceland. The backlog is spiralling out of control."
Immigration Minister Mark Harper said: "This report raises some legitimate concerns but we are taking robust action and it is working."