The Home Office is investigating allegations that five illegal immigrants had cleaned immigration service offices for years.
The Home Office said the men were stopped before working
Channel 4 News reported that five Nigerians had worked at Immigration and Nationality Directorate premises, including one for three years.
The Home Office said it would look into it "fully and as quickly as possible".
It had earlier said the men were stopped when they first turned up for work due to the "alertness" of staff.
Channel 4 News reported on Friday that the company the five worked for, Techclean, had said in a statement the cleaners had been working for months.
"Each of the five individuals has worked at the premises of the IND on a number of occasions; one of them for about three years," Channel 4 said.
Two of the men had worked for two years at IND, one cleaner for one year and one for six months, according to Techclean.
The Home Office said the home secretary had highlighted that no system could be safe.
"We understood last night that people had turned up for work for the first time and were stopped, their names checked and found not to be cleared, and the authorities were therefore called in and they were arrested," a Home Office spokesman said.
"The home secretary made it clear last night that the fact that the system had worked successfully in this way was a cause for praise for the alertness of the security guard.
"He also made it absolutely clear that no system was 100% foolproof and that there may have been occasions in the past where people had got through the system."
Shadow home secretary David Davis accused Home Secretary John Reid of misleading the public.
"Yet again we see the Home Office and now the home secretary have misled the public over a very serious breach of national and Home Office security," he said.
Techclean launched its own probe after it sent five illegal immigrants to work in the Home Office premises.
Techclean said the five men appeared to have "circumvented" safeguards.
The Nigerians were arrested in London after being sent to the Immigration and Nationality Directorate's Becket House.
"Techclean PLC and its subsidiaries have in place recruitment procedures in strict accordance with government guidelines and take the process of recruitment extremely seriously," the company's statement on Thursday said.
"This matter is subject to investigation and we will not be making any further statement at this time."
Earlier this week a director at the IND caused a row after saying he did not have the "faintest idea" how many illegal immigrants were in the UK.