The Duke of Sussex’s lawyers are to formally write to anti-monarchy group Republic to contest its claims that Harry and his brother’s charitable foundations breached Charity Commission rules.

The legal warning was highlighted in a strongly worded statement from Harry’s legal team that said his charity work was “his life’s focus” and Republic’s allegations were “deeply offensive”.

It went on to say: “The duke has not, nor has he ever, had any personal financial interest in his charitable work.”

Republic, which campaigns for an elected head of state, has reported the Duke and Duchess of Sussex and the Duke and Duchess of Cambridge’s foundations to the Charity Commission for alleged “inappropriate use of charitable funds, conflicts of interest and lack of independence”.

The claims were made after Harry and Meghan’s Sussex Royal charitable body received a six-figure grant from William and Kate’s Royal Foundation.

Harry’s fledgling not-for-profit sustainable travel organisation Travalyst also received funds from the Cambridges’ foundation, and it has received the assets of Sussex Royal, which is in the process of being wound up.

A spokesman for the Duke of Sussex’s legal team, Schillings, said: “The Duke of Sussex has always and continues to remain deeply committed to his charitable work.

“This is his life’s focus, and his devotion to charity is at the very core of the principles he lives by, and is obvious through the impact and success of his many charitable projects throughout the UK and beyond.

“To this point, it is deeply offensive to today see false claims made about the Duke of Sussex and his charitable work.”

The statement went on to say: “Travalyst (which was founded within Sussex Royal) is a non-profit organisation for which the duke receives no commercial or financial gain, as is the case with all of his charitable commitments.

“The duke has not, nor has he ever, had any personal financial interest in his charitable work. The interest has always been clear: to support others and to make a positive difference.”

Graham Smith, Republic’s chief executive, said in his letter to the Charity Commission: “The Royal Foundation gave a grant of £145,000 to Sussex Royal and £144,901 to a non-charitable organisation (Travalyst).

“In both instances it appears the only rationale for the decision was the personal relationship between two patrons, the Duke of Sussex and the Duke of Cambridge.

“Neither patrons are trustees of the Royal Foundation, so there is also a question mark over the independence of the trustees of the Royal Foundation.

“The Sussex Royal charity has since decided to close, and it is reported that they will transfer all their funds to Travalyst.

“Again, this appears to be a personal decision by a trustee (the Duke of Sussex) to fund another of his projects, rather than to ensure the funds are being used for the original purposes for which they were donated.”

The Royal Foundation had operated under the names of the Sussexes and Cambridges and was behind national campaigns like the Heads Together mental health project.

But in June last year Harry and Meghan revealed they were to formally split from their joint charity and form their own charitable organisation.

The now-defunct Sussex Royal foundation was established, while William and Kate renamed their organisation the Royal Foundation of the Duke and Duchess of Cambridge.

The Sussexes, who now live in Los Angeles after choosing financial and personal freedom over being working royals, are establishing their new foundation called Archewell.

In his letter Mr Smith called on the Charity Commission to “investigate both charities for inappropriate use of charitable funds, conflicts of interest and lack of independence”.

A Royal Foundation spokeswoman said: “The grants made to Sussex Royal were to support the charitable work of the Duke and Duchess of Sussex.

“They were fully in line with governance requirements and were reported transparently.”

A Charity Commission spokeswoman said: “We have received a complaint on this issue.

“As with all concerns raised with us we will assess the information provided to determine whether or not there is a role for the Commission.

“We have not made any determination of wrongdoing.”