A former administration officer at the major Lunar House immigration centre in Croydon told The Sun women were helped to stay in the UK in return for sex.
Anthony Pamnani, 23, said he quit after four years in disgust at the behaviour, which he claimed also included mocking any "ugly" applicants.
Those considered good looking would be seen straight away while others queue for hours, he said.
The whistleblower, who said his complaints were ignored, also told the paper that vital security checks on immigrants were not carried out.
"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 girls would be given permission to stay in the country longer than their boyfriends for no valid immigration reason.
And passport photographs of women considered too "ugly" to be allowed to stay would also be laughed at and pinned on the wall, he said.
Mr Pamnani said that in "many cases" passports were not checked to ensure immigrants had no previous convictions and were not wanted abroad.
"It was lazy because they only had to walk a few yards and swipe it through a computer reader."
He told the paper the final straw for him came when staff were given instructions to allow in more immigrants from eastern Europe at the expense of those from India.
"I lost what remaining respect I had for the job," he said.
Home Office Minister Tony McNulty said: "I have every confidence that staff within the Immigration and Nationality Directorate carry out their roles with professionalism and integrity.
"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.
"These are serious allegations and I will ensure that they are fully investigated.
"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."
Shadow home secretary David Davis said the allegations were the latest in a long list to raise serious questions about the conduct of the Home Office.
"How long is it going to take the Government to get this department sorted out?
"Over the last two years, we have seen immigration officers turning a blind eye to illegal immigrants to keep official figures down, visas and citizenship applications being rubber-stamped without proper checks, and scams in Romania and Bulgaria where people were given visas in the full knowledge their claims were fraudulent.
"Now we have extremely serious allegations of security breaches and corruption.
"Throughout that time there has been a history of cover-up in the Home Office so it is vital that the Government's investigation should be independent, public and lead to real action rather than yet more unfulfilled promises."Reuse content