An events agency led by Queen Elizabeth's grandson Peter Phillips was paid £750,000 (€867,000) in fees for staging a street party marking her 90th birthday - almost double the amount the event raised for good causes.
Mr Phillips, who leads the UK arm of Sports and Entertainment Ltd (SEL), was the driving force behind the Patron's Lunch, the centrepiece of a weekend of celebrations last June marking the Queen's milestone.
Questions had been raised about the £150 (€173) cost of the tickets for the day-long street party which saw the Duke and Duchess of Cambridge and Prince Harry join 10,000 revellers enjoying a picnic in The Mall.
Now accounts relating to the event, where the Queen and Duke of Edinburgh paraded past well-wishers, have shown the not-for-profit celebration staged by SEL UK raised £384,000 (€444,000) before tax.
Stuart Etherington, chairman of the Patron's Fund, the charity which organised the celebration, defended their contract with the events agency and highlighted how the party was enjoyed by tens of thousands of revellers in The Mall and was seen by a global audience of millions.
He added: "You can't run an event of this scale without incurring some costs.
"But our contract with SEL required them to undertake this on a non-profit basis, so that the maximum amount possible could be returned to the charity.
"This meant that the company made no profit from the event.
"In addition, we compared their costs to those of other suppliers for organising the event, arranging logistics and security, and raising commercial sponsorship, and we satisfied ourselves that they were competitive in every regard.
"The event generated nearly £400,000, all of which was given to the Patron's Fund.
"This, combined with other donations, means the Patron's Fund has been able to distribute £750,000 to the Queen's charities."
The Mall street party celebrated the hundreds of charities and organisations that have the Queen as their patron, and the total funds raised meant 290 bodies across the UK and Commonwealth were given £2,500 (€2,890) each.
The accounts, filed with Companies House, related to Patron's Lunch Events Limited, which had Mr Phillips as one of its four directors, and covered the period from April 2015 to September 2016.
It stated that turnover was just over £4 million (€4.6m) and from this was deducted "cost of sales" of £3.6 million (€4.2m) to give a gross profit of £384,000.
The report said SEL UK received £750,000 in fees, under a two-year commercial services contract, for organising the event.
Speaking in January last year when he formally launched the street party, the Queen's grandson said: "This is a not-for-profit event.
"SEL is being paid a set fee basically to take this from sign-off from the palace through to delivery of the event.''
A Buckingham Palace spokesman said: "The Patron's Lunch was an enjoyable and celebratory event which has directly benefited a large number of Her Majesty's charities.
"Questions about the organisation and financial management are rightly a matter for the Patron's Fund board of trustees."
Despite downpours which threatened to wash out the street party in The Mall last June, the day was hailed a success by the organisers and thousands of revellers.
The £750,000 raised by the event and subsequent donations helped organisations as varied as the RSPCA and the British Red Cross.