The Charity Commission is looking into a complaint against the JNF over the offer of free advice on wills in return for a bequest to the charity.
The complaint arose out of a dispute over a will which left money to a number of charities, including JNF, after the donor had consulted the KKL will service, which is run by a subsidiary of JNF UK.
But a spokesman for KKL said that the commission three years ago had expressed "satisfaction with our practices and procedures" and that it would gladly answer any queries.
The question over the KKL service stems from the death of Leeds GP Stanley Somers, who died two years ago, unmarried and childless.
In 2002, KKL helped him draw up a will, leaving the tax-free share of his estate to his eight great-nephews and nieces - a sum worth £285,000 at his death - with the residue to be divided equally among five charities.
These were: the NSPCC, the British Heart Foundation, Cancer Research UK (as the charity is now known), JNF and the Leeds Jewish Welfare Board.
Three years later, he changed the will, leaving £10,000 to each of the five charities and the remainder to the relatives.
Family sources estimate the estate overall to be worth around £750,000, which meant that if the first will had stood, most of the money would have gone to the charities - which, according to the family, was not what Dr Somers had wanted. Four of the charities - the exception being the Leeds Jewish Welfare Board - challenged the validity of the second will, but members of the doctor's family this week said they understood that it was no longer being contested by the charities.
One of Dr Somers's great-nieces, a 24-year-old medical student who declined to be named, said: "Our great-uncle loved us as if we were his own children and grandchildren.
"He was deeply distressed when he realised that the ‘residue' he left in the first will for the five charities was far more than the amount he'd left for his great-nieces and great-nephews."
She said she believed that "the whole system used by the KKL of offering wills ‘free' on condition you give money to the JNF is fundamentally wrong".
A spokesman for the Charity Commission confirmed that it had "received a complaint relating to fundraising activities by the JNF Charitable Trust and had contacted the charity to ask for additional information in relation to this matter, and are currently awaiting its response. Once that has been received, we will consider the appropriate way forward."
A KKL spokesman said that it would "gladly answer questions" put by the commission.
"In 2005, the commission's own review of KKL expressed satisfaction with our practices and procedures," said the spokesman.
"KKL is a leading communal organisation, drawing up wills and acting as the executors of wills since 1948. It is always our practice to ensure that the person drawing up a will seeks independent legal advice whenever consideration is being given to making a significant legacy to the JNF.
"In addition to the usual roles of an executor, we would point out that the KKL also performs an important role for testators who have no close family that can take care of the important and uniquely Jewish tasks of arranging the funeral, the stonesetting and the saying of kaddish."
He said that solicitors had acted for JNF and the three national charities over the contested estate, adding: "Proceedings were not issued, and following a recent meeting, all the parties are hopeful of resolving the dispute."
Information about legacies posted on the JNF's website says that the solicitor who heads KKL's wills service "combines expert legal knowledge with a deep understanding of Jewish concerns. And his time and advice is free if JNF benefits from a legacy."
The JNF gained £532,000 from legacies, according to its latest published accounts in 2006.
According to the Charity Commission's published guidelines, there is no objection to a charity offering to meet the cost of a preparation of a will "in the belief that the charity will receive a legacy".
But if it does, the commission "strongly advises that charity employees should never become involved in drafting an individual's will."