A college is under investigation following claims it was illegally selling English language certificates to help hundreds of immigrants settle in the UK.
Students at the Manchester-based Oxford College of Management Sciences appeared to have "little or no knowledge of what, when and where they had studied" and could buy the certificates for between £250 and £450, an investigation by The Times found.
UK Border Agency (UKBA) officials said anyone applying for citizenship who claimed to have an English language qualification from the college had been automatically refused since April.
Lin Homer, UKBA chief executive, said: "I take allegations regarding any abuse of the immigration system extremely seriously and will ensure that action is taken by officers on intelligence received. This Government is committed to cracking down on this kind of abuse."
The accusations will fuel concerns that bogus colleges are providing the English for Speakers of Other Languages (Esol) certificates, part of the requirement for UK citizenship, as they exploit weaknesses in the UK's immigration controls.
In December last year, exams regulator Ofqual raised concerns that immigrants could use the certificates as a means of applying for permanent residency in the UK.
It warned that the certificates were "being issued with virtually no training or learning taking place and with insufficient rigour in the assessment process".
Ms Homer added: "This college is not accredited and since April we have automatically refused any applicant with an English language qualification from this college.
"We have referred details of this college to the Office of the Qualifications and Examinations Regulator and City of Guilds to investigate as a matter of urgency.
More than 203,000 immigrants were granted British citizenship last year, a 58% rise on the previous year and the highest number since records were first published in 1962, Home Office statistics showed.