More than 70,000 people are waiting to hear if they can become British citizens, BBC News has learned.
A record 140,795 people were granted citizenship in 2004
The Home Office confirmed it had been overwhelmed by people who applied before 1 November, after which citizenship tests became compulsory.
Home Office minister Tony McNulty said his staff were working hard but would be unable to clear the backlog until May or June 2006.
He said: "I can only apologise for the disruption to people's lives."
Mr McNulty added: "Resources are being put in place to deal with it as quickly as possible."
Prospective new citizens applying after 1 November 2005 will have to take a 45-minute test - covering government, society and practical issues and costing £34 - at one of 90 centres across the country.
But those who applied before the deadline need only demonstrate a working knowledge of the English language.
The number of people granted citizenship reached a record 140,795 in 2004 - a rise of 12% on 2003.
Applications for citizenship ran at about 40,000 a year during the mid-1990s until, in line with migration trends, they began rising in 1998.
While the number of people granted citizenship grew, the rate of new applications fell slightly during 2004, probably due to the new English language requirements, which caused some people to wait.
Just under half of all applications were granted on the basis of residence in the UK.
Some 29% of new citizens were accepted on the basis of marriage to a British resident, while about a fifth were children.
People born in Asian or African countries accounted for 40% and 32% respectively of all applications, the principal nationalities being Pakistani, Indian and Somalian.
Almost 60% of people born abroad living in the UK take British citizenship within six years, according to figures from 2004.