On the social media pages of all of its more than 150 care homes, Care UK says it is "aware of unsolicited and fraudulent job offers being made, allegedly from our care homes".

"If you are in any doubt of the legitimacy of an offer; we never ask you for money as part of a job application," Care UK adds.

The scam - revealed by Sky News - has emerged as the government tightens immigration rules to cut the number of migrants entering the UK.

In the year to June, more than a third (37%) of long-term work visas went to care workers.

Amy, who lives in South Africa, was a victim of the scam while trying to get a visa to move to the UK, where her boyfriend lives. The 25-year-old's name has been changed to protect her identity.

After seeing an advert offering roles within healthcare, Amy contacted what she thought was a recruitment agency, Zidaan Consultancy, in July.

She received an email stating a five-year certificate of sponsorship and visa processing would cost £4,500.

She decided to apply for care work because it was "the sector where most jobs were available".

Amy paid an initial £500 "registration fee" and was advised a further £4,000 would be due once her certificate of sponsorship (a document required in order to be granted a visa) was issued.

Zidaan Consultancy then arranged a video interview with someone Amy was told was a representative of Care UK.

In September, she received another email, forwarded from Zidaan Consultancy, saying her interview was successful.

There was a job offer for the full-time role of healthcare assistant on a salary of £23,000 a year.

This message was originally sent by a woman whose email signature stated she was "recruitment director administrator" at Care UK. The same text was sent to 15 other people.

Amy contacted Care UK directly after she was given no further details on how to progress with a visa. She discovered Care UK had no record of her and the woman who had signed the job offer email was not an employee.

"I was very, very upset because it was such a long process to try and get the visa and then when I thought it was going to happen I was so happy," Amy said.

"Now, all of a sudden it wasn't even true. It was like I have to start from the beginning."

Care UK says it has reported at least one company to the Home Office and it is working with the police on a separate investigation.

Leah Queripel, human resources director at Care UK, told Sky News: "It makes me very angry because they're using Care UK's name but they're also trying to extort money from people and that is under our name.

"This does need to be looked at from a government perspective and not for companies to have to be relied on to fix it."

Under the government's new immigration rules, all care firms will have to be registered with the regulator, the Care Quality Commission, in order to get sponsorship licences, which allow them to recruit abroad.

But these new rules will do nothing to stop this kind of scam, where people fraudulently impersonate legitimate firms in order to illegally make money from people who want to move to the UK.

The Gangmasters and Labour Abuse Authority (GLAA) previously revealed to Sky News that exploitation of overseas care workers is its "number one priority".

Marley Morris, associate director for migration, trade and communities at the Institute for Public Policy Research, told Sky News that a "more effective approach is actually targeting the abuse directly by ensuring higher standards, by resourcing the labour inspectorates properly, by ensuring that people are able to report abuse in a way that protects them".

"Those, I think, are the measures that are going to deal with the issue rather than measures that are really targeted at just bringing down the overall numbers," he added.

Zidaan Consultancy is still advertising vacancies for international recruits on its website.

Care UK says Zidaan Consultancy is not a company it has ever used.

Sky News has repeatedly emailed, messaged and tried to call the director of Zidaan Consultancy, Izzah Zidaan, to ask him to formally respond to these allegations. There has been no response at the time of publication.

A Home Office spokesperson said: "We do not tolerate abuse in the labour market and where we identify exploitative practices are being undertaken, we take action.

"The GLAA also work with other law enforcement agencies to identify illegal working, including incidents being reported to Action Fraud."