The Home Office is investigating a claim that immigration officials have granted visas to foreign nationals in exchange for sex.
The immigration worker alleged slack security
The Sun reported the allegation by a former administration officer at the Lunar House immigration processing centre in Croydon, south London.
He also alleged security checks were lax, making it easier for potential terrorists to sneak into the country.
A Home Office minister said he was confident staff were professional.
"I have every confidence that staff within the Immigration and Nationality Directorate carry out their roles with professionalism and integrity," Tony McNulty said.
"There are clearly established systems for staff to raise any concerns that they may have with working practices within their team and to take the issue further if they feel it is necessary."
Mr McNulty added that the allegations were "serious" and he would ensure they were investigated fully.
"Until the outcome of that investigation is known, it would be inappropriate for me to comment further but clearly I will not condone this type of behaviour amongst staff."
The Sun said that a former administrator, Anthony Pamnani, had alleged that attractive applicants were seen first.
"One girl came in and told us an admin officer had visited her flat and they had slept together. She got indefinite leave to stay," he said.
Brazilian women were given longer stays in the country than their boyfriends for no reason, and passport pictures of women considered "ugly" were pinned up for the amusement of male staff.
Mr Pamnani said he had left his job in disgust at staff too lazy to check passports for criminal convictions, and instructions to let in more eastern Europeans at the expense of south Asians.
Keith Best, of the Immigration Advisory Service, a charity helping refugees, told BBC News he had heard various allegations in the past, but nothing that could be substantiated.
"We have heard allegations of all sorts of things - sexual favours, bribes. Whether they are true or not is another matter. Some people might be complaining more about the refusal [of their application] than what has actually happened."
But he said the latest allegations, and other allegations over immigration procedures, showed that decisions on visas should be made by a body independent of government, as happened in Canada.
"Is anybody going to have full confidence in an internal investigation? You need something like the Independent Police Complaints Commission looking at things like this or you have to have it completely separate of government."