Immigration Minister Mark Harper has said the increase in the number of outstanding immigration and asylum cases raises "legitimate concerns".
In a statement, he said:
This report raises some legitimate concerns but we are taking robust action and it is working.
Every day it gets harder to live illegally in the UK - we are tracking people down and taking action against them.
We are restricting access to benefits, free healthcare and financial products, and businesses can be fined up to £10,000 for every illegal worker they employ.
We are winning more deportation cases in the courts, exceeding visa processing targets and have introduced interviews to test whether foreign students are genuine - all of which are praised in this report.