An investigation was launched after two immigration workers were suspected of being members of the far-right British National Party, it has emerged.
The UK Border Agency said the pair were employed in an immigration removal centre until a BNP member list was posted on the internet, last November.
An agency spokesman said one resigned and the other has been suspended.
He added anyone working with the agency must sign a declaration stating they are neither BNP nor Combat-18 members.
“There is no place for racism in the immigration system,” the spokesman said.
The two workers were employed by a private contractor on behalf of the Home Office.
The investigation, which is looking into how the two staff may have slipped through the screening net, began following the resignation of one of them, last November.
Most people held at immigration removal centres are failed asylum seekers whose legal rights to stay in the UK have been exhausted.
The centres house them until they can be deported, which in some cases takes months.
Often, they are in breach of immigration regulations or thought to be likely to abscond while awaiting a decision on their case.
Simon Darby, deputy leader of the BNP, said he could not confirm that the two workers were party members.
The internet list showed the full contact details of BNP members including home phone numbers and e-mail addresses.
Among them was a Merseyside police officer, who was suspended from duty while an investigation was undertaken. The force specifically bans all its officers from membership of the BNP, Combat 18 and the National Front.
Mr Darby said he did not know how many members’ jobs had been under threat since the publication of the list,but the party was offering legal support to those affected.
The BNP, an anti-immigration party, has won council seats in recent years, and took a London Assembly seat last May.