Chief inspector of the UK Border Agency John Vine was asked to evaluate how well UKBA had handled the backlog of thousands of unresolved immigration cases.
Mr Vine said he believed little had been done to try to resolve the cases before they were passed over.
He said: "Through the inefficiency and delay of the agency, those who would otherwise have faced removal will have accrued rights to remain in the UK."
Mr Vine also criticised "poor" customer service, and said that a lack of resources meant that deadlines were often missed, even when legal action was threatened.
The report said: "The issue of limited resources also created a significant impediment to case clearance.
"As a result, timescales given to applicants or their representatives about the resolution of cases were frequently missed, even where litigation was being threatened."
More top news
A grandma who became an online hit when her grandson started tweeting pictures of her balancing things on her head has gone viral - again.
Raincoats and wellie boots are the order of the day this half term, as forecasters predict a gloomy school holiday.
China’s one-child policy has created a shortage of women in the country - and the nation's men have started to look elsewhere to find a wife