Tate Members was established in 1958 as Friends of the Tate, with the charitable purpose of supporting the Tate in improving public knowledge, understanding and enjoyment of art. Last year it contributed £7.2m to the Tate’s funds.
The Tate operates as a separate charity, but this weekend the Tate Members board sent members a letter outlining proposals to merge the two organisations, adding its own council into the Tate’s existing governance structure.
The proposals will be subject to a vote by members at an extraordinary general meeting on 26 February.
But Michael Paraskos, a member of the Tate Members charity and an arts lecturer at a number of London universities and colleges, warned the move could leave those who belong to Tate Members unrepresented.
"Although the charities work together at the moment, the members’ charity works from the perspective of the visitors and their experience – and if it became part of the gallery, it would have a different perspective," he said.
He said the move was deeply troubling and undemocratic, and warned it would create "a concentration of power in the hands of a few".
Under the proposals, the membership council, the elected directors and trustees of the members’ charity, would become a committee within the Tate charity’s existing board.
Paraskos said the income raised by Tate Members was "the only independent leverage on this behemoth of a gallery to maintain concern for the needs of its long-term supporters". He expressed concern that a merger meant the money would "be lost into the general budget of the Tate, along with the Tate Members’ reserves of about £5m".
But a Tate spokeswoman said the move would give members more say in the running of the gallery.
"The proposed change would enable members to be more closely integrated in the decision-making process at Tate, giving them more influence, not less, because they would be represented on the councils of each of Tate’s four galleries and would, for the first time, have representation on the Tate board."
She said the consultation process with members had been fully open and democratic, and any member would be able to stand for election to the Tate Members council.
She said the Charity Commission had been informed about the proposals.
A commission spokesman said: "We will be in contact with the charity to discuss this proposal further and examine the issues raised by this proposed merger."