"The UK Border Agency's same-day visa service are being sold by unofficial agents," the BBC reported.
The agents, some based abroad, sell the slots - for up to 200 pounds - to foreign nationals unable to secure one themselves from the UK Border Agency (UKBA), it said.
It is claimed the online booking system crashes nightly, as people try to avoid the long delays of applying by post. Foreigners already living and working in the UK can renew their visa by post, but they have to surrender their passport while the application is processed - making travel impossible.
A backlog of outstanding applications has seen the time taken for a decision rise from the advertised four weeks for a work visa to more than six months in some cases.
As a result, demand for the premium same-day service at one of seven UKBA offices has grown rapidly, the report said.
These appointments are released on the UKBA website each night at midnight, with cancelled appointments re-released sporadically.
But the BBC said it has heard from dozens of people who complain that the site simply cannot cope with demand.
Paul, an IT worker who lives in London but is originally from New Zealand, found that every time he logged on at midnight the site crashed.
The report said a cottage industry has developed where people advertise online, promising that they will spend hours trying to grab an appointment.
The BBC also contacted a dozen such individuals and the price they charge ranges from 100 to 200 pounds.
This is on top of the cost of the premium service itself, which varies, depending on which visa is applied for, between 300 and 400 pounds and is paid for at the appointment.
The BBC secretly recorded several people offering to sell appointments, which is in breach of UKBA rules.
One man, who is based in Turkey, said that he had booked more than 50 appointments in two months.
Part of the attraction of these unofficial agents is that they are cheaper than registered immigration lawyers or advisers, who are granted a separate allocation of premium appointments.
They charge an average of 800 pounds on top of the premium fee.
A spokesperson for the UK Border Agency said: "We are aware that a small number of individuals and organisations are attempting to speculatively or block-book appointments to sell on for profit and we are changing the system to prevent this type of abuse.
"Anyone found to be misusing the appointment service in this way has their access immediately withdrawn," the spokesperson added.