Some 14,000 applicants, growing at a rate of 700 a month, who have been refused the right to stay are still pleading with the UK Border Agency (UKBA) to reconsider.
And an additional 2100 cases – some dating back a decade – were still waiting for an initial decision at the time of the inspection. The UKBA said these had since been cleared.
The backlogs were discovered by the Independent Chief Inspector for Borders and Immigration John Vine as part of an inquiry into applications to remain in Britain on the basis of marriage.
"This is completely unacceptable and I expect the Agency to deal with both types of case as a matter of urgency," Mr Vine said.
He added that he was "surprised" to discover the UKBA was also failing to check whether applicants earn enough to look after themselves without having to rely on state handouts.
Meanwhile, a separate inspection into how the UKBA and Border Force deal with criminals at ports, such as Heathrow Airport, discovered that the policy of swift removal was being rendered ineffective by the numbers of immigrants claiming asylum.