Multiple failures at the UK Border Agency (UKBA) are ruining the government's reputation, the home affairs committee has concluded.
A report by Keith Vaz's select committee found the agency, whose temporary suspension of full checks last summer nearly led to home secretary Theresa May's resignation, was continuing to fail in its basic functions.
Six years after it emerged that the UKBA had failed to deport over 1,013 foreign national prisoners, who had instead been released between 1999 and 2006, MPs noted that the UKBA had still to deport over 600 of them.
A further 520 foreign national prisoners were released in 2010/11 and have been given leave to remain in the country.
The UKBA's failure to address the problem may be partly due to the huge backlog of asylum cases it is struggling to cope with. It has reduced this from up to 450,000 to around 17,000 since 2006, but more are continuing to be discovered.
"UKBA appears unable to focus on its key task of tracking and removing illegal immigrants, overstayers or bogus students from the country," committee chair Mr Vaz said.
"The so-called 'controlled archive', the dumping ground for cases where the UK Border Agency has lost track of the applicant, will take a further four years to clear at the current rate of resolution. This is unacceptable."
MPs also noted with dismay UKBA's 119,000 files on 'lost' applicants, a £9.1 million iris scanner initiative scrapped after just five years and ongoing question-marks over the e-borders scheme.
The agency then found itself in the public eye last summer, after it emerged border checks were being suspended.
Reduced checks were introduced following concerns in airports and in government at the length of the queues at immigration checks.
Ms May insisted the shift took place without her explicit sanction. The UK Border Force was eventually separated from UKBA after the departure of its head, Brodie Clark, who had insisted to MPs that he was no "rogue officer".
"Following the border controls saga we now have two agencies instead of one," Mr Vaz added.
"We are hopeful that the UKBA will now concentrate fully on the issues that are causing so much concern to the public and to parliament."