The chief inspector of immigration said the latest backlogs relating to requests to settle through marriage were “unacceptable”.
The backlogs included people waiting a decade to hear if their partner had permission to live in the UK.
The Home Office said the UKBA was already taking action to deal with historic backlogs.
The marriage or civil partnership-related backlog is thought to be the 10th category of unresolved cases that has emerged in the past two years.
Inspectors looked at how well the agency was handling applications from people from outside the European Economic Area who have a partner already in the country, such as a husband or wife who is British.
John Vine, the Independent Chief Inspector of Borders and Immigration, said the majority of decisions were reasonable and caseworkers checked whether applicants were known criminals.
But he said that his team had found a backlog of 14,000 cases where the UKBA failed to deal with requests to reconsider decisions after an initial rejection.
When inspectors visited a UKBA office in Sheffield they found a further backlog of 2,100 “complex” temporary migration cases. These included 180 marriage entry or settlement applications which had not received an initial decision. Some dated back to 2003.
One inspector said the unresolved applications had been effectively “dumped” on the Sheffield team after being transferred in a box from the UKBA’s Croydon offices. Officials have since dealt with these cases.
At the time of the inspection last year, the marriage-related backlog was growing at a rate of 700 cases a month. Caseworkers told inspectors they could not deal with them until they had received new policy guidance.