Four years ago, the trustees of Tayside NHS Board Endowment Fund agreed to allow Tayside University Hospital Trust, of which they were also board members, to use its funds to buy the uniforms.
The Office of the Scottish Charity Regulator looked into the matter after it received a complaint from a member of the public, and concluded that it was a legitimate use of funds. But chief executives group Acevo and the Scottish Council for Voluntary Organisations have since raised concerns.
In a letter sent to the charity by the OSCR, the regulator's investigator confirms that it was within the trustees' powers to sanction the "unusual" use of the charity's funds. But he criticises the endowment fund for sanctioning the uniform purchases at a meeting attended by only two trustees.
"Greater care should have been taken to ensure it could be shown that the process had been properly considered," the letter reads.
Stephen Bubb, chief executive of Acevo, said: "It was a serious misuse of charity funds. It is a short step from paying for uniforms to subsidising pay."
Stephen Maxwell, associate director of the SCVO, said the OSCR conclusion was "a very curious finding".
George Hepburn, chief executive of the Community Foundation serving Tyne and Wear, said: "Trustees need to ask hard questions about whether this is the best use of charity money, rather than whether it is legal."
The University Hospital Trust has since been absorbed into Tayside NHS Trust. The 22 trustees of the fund are on the board of the NHS trust.