Thousands of UK nationals have bought fake degrees from a multi-million pound "diploma mill" in Pakistan, a BBC Radio 4's File on Four programme investigation has found.
Buyers include NHS consultants, nurses and a large defence contractor.
One British buyer spent almost £500,000 on bogus documents.
The Department for Education said it was taking "decisive action to crack down on degree fraud" that "cheats genuine learners".
Axact, which claims to be the "world's largest IT company", operates a network of hundreds of fake online universities run by agents from a Karachi call centre.
With names such as Brooklyn Park University and Nixon University, they feature stock images of smiling students and even fake news articles singing the institution's praises.
According to documents seen by BBC Radio 4's File on Four programme, more than 3,000 fake Axact qualifications were sold to UK-based buyers in 2013 and 2014, including master's degrees, doctorates and PhDs.
A trawl through the list of Axact UK buyers, seen by the BBC, reveals various NHS clinical staff, including an ophthalmologist, nurses, a psychologist, and numerous consultants also bought fake degrees.
A consultant at a London teaching hospital bought a degree in internal medicine from the fake Belford University in 2007.
The doctor - who had previously been disciplined by the General Medical Council (GMC) for failing to report a criminal conviction - told the BBC he had not used the certificates because they "had not been authenticated".
An anaesthetist who bought a degree in "hospital management" said he had not used the qualification in the UK.
And a consultant in paediatric emergency medicine, who bought a "master of science in health care technology", claimed it was an "utter surprise" when the BBC told him it was fake.
There is no suggestion any of these clinicians do not hold appropriate original medical qualifications.
The General Medical Council (GMC) said it was up to employers to verify any qualifications additional to medical degrees.
But Higher Education Degree Datacheck (HEDD) chief executive Jayne Rowley said only 20% of UK employers ran proper checks on applicants' qualifications.
And while purchasing a fake diploma was not illegal in the UK, using one to apply for employment constituted fraud by misrepresentation and could result in a 10-year prison sentence.
"[The GMC] are correct in that [doctors] are licensed to practice medicine if they have a legitimate medical degree. But [by buying a fake degree], they have still committed fraud and could still be prosecuted," she said.
Danny Mortimer, chief executive of NHS Employers, said all NHS trusts operated rigorous primary checks.
Verification was "achieved through a variety of channels" and fraudulent activity would be reported to police, he said.
In 2015, Axact sold more than 215,000 fake qualifications globally, through approximately 350 fictitious high schools and universities, making $51m (£37.5m) that year alone.
Former FBI agent Allen Ezell, who has been investigating diploma mills since the 1980s, said: "We live in a credential conscious society around the world.
"So as long as paper has a value, there's going to be somebody that counterfeits it and prints it and sells it.
"Employers are not doing their due diligence in checking out the papers, so it makes it work. It's the damnedest thing we've ever seen."
'Very serious issue'
Defence contractor FB Heliservices bought fake Axact degrees for seven employees, including two helicopter pilots, between 2013 and 2015.
One of these employees, speaking anonymously to the BBC, said soon after he had been given a contract to work on the Caribbean island of Curacao, the local government decided all those working in the territory had to have a degree.
"We looked into distance learning, and contact was made with this online university. It was just something that needed to be done to keep working in the country.
"Everyone knew they were not bona fide. But no-one had a problem with it."
Parent-company Cobham held an internal investigation into the incident, but decided the purchase was a "historic issue" that "had no impact upon the safety of any of its operations or the training of any individuals in the UK or elsewhere".
"Procedural and disciplinary actions have been taken to address all the issues raised," it added.
But MP James Frith, a member of the Education Select Committee, said the decision was a "very serious issue".
"I am amazed that a business would put itself and its very existence at risk by having fraudulent qualifications to, by the sounds of it, get into a new market."
Following a New York Times expose in 2015, Axact's chief executive was arrested and an investigation launched by the Pakistani authorities.
Senior manager Umair Hamid was sentenced to 21 months in a US prison in August 2017 for his part in Axact's fraud.
Yet the Pakistani investigation has ground to a halt amid claims of government corruption.
Allan Ezell said Axact continued to launch new online universities all the time - and had now branched out into extortion and blackmail.
"It's a whole new game," he said. "Normally a diploma mill is finished with you by the time you get your degree. That's just the beginning now.
"You get a telephone call that looks like it's coming from your embassy or local law enforcement, threatening to arrest or deport you unless you get some additional documents to help support the phony diploma you already have. We've never seen that before."
Cecil Horner, a British engineer based in Saudi Arabia, was still getting threatening calls from Axact agents after paying nearly £500,000 for fake documents.
Mr Horner's son Malcolm said he believed his father, who died in 2015, had bought the qualifications because of the fear of losing his job.
"It makes me so angry," he said.
"It's unfathomable these websites still exist and they can't be shut down."
Action Fraud, the UK's national cybercrime reporting centre, said it did not have the power to close fake Axact websites but instead had to provide evidence to domain registries and registrars, which could take months.
MP James Frith said he was "staggered" by the "aggressive tactics" used by Axact and would ask the Education Selection Committee to look into the issue.
The Department of Education said HEDD was taking a proactive approach.
"Degree fraud cheats both genuine learners and employers, so we've taken decisive action to crack down on those seeking to profit from it," a spokesman said.
Axact did not respond to a request for an interview from the BBC.
File on 4: Degrees of Deception is on BBC Radio 4 on Tuesday 16 January at 20:00 GMT and on the iPlayer.