The partner and the brother of a banker are locked in a High Court battle, with the man describing the woman as a "call girl" who conned the late businessman out of £4million from his £18m fortune.
Danny Truell made his fortune with Goldman Sachs, before becoming the UK's best-paid charity boss, earning £3million a year as chairman of the Wellcome Trust.
In later life, he was diagnosed with motor neurone disease and multiple sclerosis.
When he died at the age of 55 in 2019, Mr Truell was living in a "modest and poorly furnished" basement flat in Clapham, south London, despite having millions to his name.
His will put most of his £14m fortune into a charitable trust, but another £4m is now at the centre of a bitter fight between his brother, Edi Truell, and his partner, a Polish former nightclub boss, Magda Zalinska.
Miss Zalinska had been in a relationship with Mr Truell and, in the seven years before he died, was handed his interest in the flat in Clapham where she lived, plus over £2m cash, while spending and withdrawing from his bank account another £1.3m, says his brother.
But despite accepting they had a sexual relationship, lawyers for Edi Truell, as an executor of his brother's estate, claim Miss Zalinska was a "call girl" whose influence over the banker resulted in him handing over cash and property.
She had also used her Southwark nightclub business, which she told Danny was struggling, as a "front" in order to "extract" from him huge sums of money, which have since disappeared, it is claimed.
But Miss Zalinska denies the accusations, insisting she was in a committed and loving relationship with Mr Truell, and pointing to the fact he called her his "partner and dependent" in his will.
The case is set for a two-week trial at the High Court next year, but in a hearing this week a judge, Master Francesca Kaye, was told that it revolves around the nature of the "strange relationship" between the pair.
Danny Truell was born in Malaysia to an Army family and educated at Oxford before moving into finance, where he had a reputation for bold left-field decisions which made a fortune for his employers.
First, he invested heavily in tech stocks after the dot-com bubble burst, before making a mint for the Wellcome Trust by anticipating the 2008 stock market crash, selling stakes before and then ploughing the charity's cash into investments as prices collapsed.
His success allowed the charity to transform its finances, so that it doubled its science funding to £1 billion a year. It made him the best-paid charity worker in the country, earning up to £3m a year.
But whereas he had been highly successful in his professional life, he lived a very different life at home in his flat in Aukland Road, Clapham, where he struggled with alcoholism and poor health.
Miss Zalinska claims to have met Mr Truell at the end of his previous marriage and started a relationship with him in 2004, and it is accepted that she moved into a flat he owned in Clapham, by 2016, the court heard.
However, Mr Truell's 60-year-old brother Edi - also a City high-flyer - and former solicitor John Rayner Hatchard, as executors of his estate, claim evidence casts doubt on the "quality" of the relationship.
Their barrister Edward Hicks told the judge that Mr Truell had two "very separate" lives - one with his family and the other with Miss Zalinska, with whom he had a "very strange relationship".
He said a crucial question at the trial will be whether Mr Truell and Miss Zalinska were in a relationship akin to "husband and wife" or whether it had been "marked by promiscuity", which he did not know about.
"Is this a lady who is presenting a false picture of her relationship to Danny?" he said.
"We have evidence that she was in relationships with other people."
For Miss Zalinska, Amit Karia said the allegation was that she had been a "call girl" - but pointed to a portion of Mr Truell's will, in which she is described as his "dependent and partner".
"The documents which the deceased created at the time would suggest they were in a committed relationship," he said.
"He called her a dependant partner in the will."
The true nature of their relationship was also shown by the fact that Mr Truell also left her cash and belongings worth more than £1m in the will, he added.
But Mr Hicks said many of the transactions between the pair before Mr Truell's death needed to be explained.
He claims the transfers - which included £1.34m paid between 2013 and 2018 to a company, Magfos Ltd, through which she operated her London nightclub, Southwark Rooms - were tainted by the nature of the relationship.
Another £915,000 went to her directly in electronic transfers, £1.37m on spending and withdrawals using his debit card, while he also transferred to her his interest in the £750,000 flat she had previously rented from him in Clapham.
He said the executors are alleging "undue influence" and, in relation to money given to her company, "misrepresentation" against Miss Zalinska.
"The sheer volume of money that goes out of the account to Magda is staggering," he told the judge.
"It's hundreds and hundreds of thousands of pounds and it accelerates as he becomes more ill.
"What was this money being used for? Working out what happened is not going to be easy."
He added: "We say that she had had very large amounts of cash. We don't know what they were used for."
He said Miss Zalinska would also need to explain why money was given to her business and what it was used for - if it was used for the business at all.
"Our case on Magfos is that this really was a front through which money was extracted from Danny," he said.
Mr Karia said Miss Zalinska had explained why the money had gone to her, claiming that Mr Truell had given it for the upkeep of Lavender Gardens, to fund renovation projects in Poland and for investments.
But Mr Hicks said Lavender Gardens was in a state of disrepair, while no further detail of the investments had been forthcoming. There was "nothing to show" for any property deals in Poland, he added.
The full trial of the £4m claim against her is expected to go ahead next year. It could be followed by a claim by Miss Zalinska for further provision from Mr Truell's estate, the court was told.
