It said the integrity of visa applications was taken very seriously.

A legal practice based in London called Keyard Law Associates - who call themselves immigration law specialists - advertised in a Chinese language newspaper offering to "design plans" for immigrants on low incomes to apply for highly-skilled migrant visas.

The Chinese undercover journalist with Radio 5 Live's Donal MacIntyre programme answered the advert and met Mr Wong, an adviser with the firm, at his office in London's Chinatown.

She posed as a foreign national who was not earning enough to qualify for a visa to allow her to stay and work.

"The key point for you is finding a company in China which is willing to give you documents to prove your income," he said.

"But you can't just find a company. You also need someone who can pick up the phone and be able to confirm it."

After advising the journalist to find a company in China prepared to tell the Home Office that she had worked there, Mr Wong went on to explain what salary she would need to make up.

"It can't be too high. If it's unreasonable, they'll get suspicious. We'll make it just enough, maybe a little bit higher than they're asking for."

He told the reporter he had lost count of the number of cases he had dealt with and boasted that they had all been successful.

In another case, the investigation found an advert in a Chinese language newspaper for a UK-registered firm called Berkeleys solicitors, based in Croydon, in south London, which offered "technical manoeuvring" in the face of the Immigration Agency's "despotic rules".

The undercover journalist met Mr Lu, a consultant with the firm at his office in London, and she posed once more as a migrant without the necessary salary to qualify for a highly-skilled migrant visa.

He advised her to start up a fake company to make it look as if she was earning more than she was.

"You can start up a consultancy company. Then you pay in some money to your account and take some of the money out and there'll be a cash flow," he told her.

"You just use this company as your extra income."

He advised the journalist to pay more tax through the fake company to make it look as if she was earning enough to succeed in her application.

"The company's just for your visa application to show the Immigration Agency."

Mr Lu said he would charge £1,500, as well as extra accountants' fees to fake tax returns.

He later told the programme there had been a misunderstanding and he would never have advised the journalist to do anything illegal.

After being told about the programme's findings, Berkeleys Solicitors said Mr Lu had been suspended while they investigated and they have placed a retraction in the Chinese language newspaper.

Mr Wong, the adviser at Keyard Law Associates, said he was deeply shocked and surprised by the allegations and told the programme it was practically and technically not feasible to falsify salaries.