Other former members have also come forward to allege that "masters" at the healing centre, a registered charity, tried to control clients' lives and pressurise them to make donations.
The claims have been rejected by Ki Health International, which said it was being targeted by disgruntled former members.
Its supporters claim that it is simply a healing centre using oriental "energy" treatments to alleviate ailments ranging from epilepsy to ME.
However, the Charity Commission confirmed that it was considering the claims.
The family of the former client alleges that the charity "brainwashed" him into making donations amounting to £300,000. They claim he was pressurised into handing over about £100,000 to the charity and £200,000 to a private bank account in South Korea.
A person close to the family said that he underwent a "personality change" after he started going to the centre. "He was brainwashed," the source said. "It was subtle but he was brainwashed."
The family has written to Ki Health threatening legal action unless the money is returned. The parents of the man, from London, who does not wish to be named, claim in the letter that he was the victim of "undue influence" when he went to the healing centre after developing ME (myalgic encephalopathy or chronic fatigue syndrome).
The charity, which has been in Britain for 10 years, denied that it exerted "undue unfluence" on any of its clients.
Emilie Weston, 31, a co-director of the charity with Soon Tak Oh - a Korean Ki "master" she married in 2002 and from whom she is now understood to be separated - admitted that an allegation had been made by the family of a man in his thirties who donated a six-figure sum.
"We treat hundreds of people a year," said Ms Weston. "If some want to donate to a charity that's up to them. The family got involved and it all became unpleasant."
Therapists at the centre near Piccadilly Circus use techniques developed by a Korean couple jailed in 2000 for swindling their followers out of almost £45 million.
Mo Haeng Yong and Park Gui Dal, founders and grandmasters of the Chojonhoe (Heaven's Gathering) organisation, were jailed in Seoul for eight and five years, respectively. They had told their adherents that the world would end but they would be spared if they donated money for a shrine.
Former clients of Ki Health International told The Sunday Telegraph that many of the treatments were effective but they felt pressure to pay large sums of money for "ancestral healing" - at £1,000 per course - to get rid of "negative energy" from their forebears, and to make extra donations.
The Ki masters use breathing techniques - including hissing and belching - to transmit "Ki energy" into the body, opening "major energy pathways" by pressing points on the body.
The charity said that it had no "financial or managerial connection" with the organisation in Korea but confirmed that Park Gui Dal visited the centre in August last year, at Ki Health's invitation, and talked to clients.
At the centre recently there were envelopes on the reception desk inviting donations ranging from £100 for a "window of clarity" to £25,000 for a "positivity pillar".
Christiana Webb, 55, an IT worker from north London, went to Ki Health after visiting its stand at an alternative health fair in London in 2001 "with the intention of trying different things out".
She claims that she paid thousands of pounds over a two-year period.
She was asked for large sums to "sponsor ceremonies" and for "ancestor training". The masters said the donations would help her ancestors "move towards the light" and benefit her whole family, and they suggested she took out loans or use credit cards to pay.
Ms Webb spent several hours a day, three days a week at the centre and regularly took part in all-night ceremonies. She went to the grandmasters' Korean headquarters to complete her master training and was shown their spacious accommodation at Daerachun.
Ms Webb left the centre because she felt she and other clients were being manipulated. She considered trying to get her money back but was worried that a legal battle would increase her debts.
Other former clients said that Ki Health defended the grandmasters despite their fraudulent past. In an email to one former client, Ms Weston refers to "the terrible situation our grandmasters have had to endure".
In another email, Mr Oh responds to criticism of the organisation by threatening a former client: "You have to know God is watching you. You will pay a very high price for it in this world and after death."
Anna Zimmerman, 38, a Cambridge graduate who is a hypnotherapist and part-time lecturer, left the centre in 2004 after two spells there. "I joined because I had been in a bad relationship and wanted to sort my head out," she said.
"Right from the start, we were told that donating money was an important way of 'showing our beautiful mind', a phrase constantly echoed by the masters... I believe this is a standard method when manipulating people.
"When I questioned the expense, I was told that money was a way of showing our positive intent to heal ourselves... Ms Weston said that I could take out a bank loan."
Many of the initial treatments were helpful, she said, but the masters insisted they were "uniquely beneficial" and this put emotional pressure on them to commit to the more expensive programmes, including "round after round of ancestor training".
"During my second stint of training, my grandma and aunt died and I was told that it was critically important that I immediately embark on training dedicated to each of them - very expensive, of course."
Former clients said that they were also asked to bring large amounts of food, flowers and wine to the centre.
Ms Weston said the critics were engaged in a "conspiracy" to damage the centre. The only payments made to South Korea were small sums for doors for the London centre and uniforms for staff.
In a statement, Ki Health's solicitors, Carter-Ruck, said: "Any personal transactions made to Korea by Ki Health's staff or clients for any other circumstances are personal, outside of our client's remit and have nothing to do with the services of Ki Health International."
The statement added that it was the London man's "positive experience of the Ki Health healing techniques" that led him to "volunteer the donation". Many clients praised the treatments and said that they were not put under pressure to make extra donations.
Prachi Ranade, 25, from north London, said her epilepsy had improved from "severe" to "mild" at a cost of almost £5,000 in the past four years.
Caroline Dale, 60, who has been attending Ki Health for 18 months, said that the treatments had helped to raise her energy levels and to "heal family relationships".
Marie Park, 55, who lives in Portugal and was treated for ME at Ki Health from 1999 to 2003, said: "I recovered my health. I was not put under any pressure to do extra training or make donations."
A spokesman for the Charity Commission said: "Concerns have been raised about Ki Health relating to donations made and to the charity's financial controls.
"We are considering what role there may be for us in connection with this matter."
