The Turner Prize has been criticised over its sponsorship from a businessman who was in favour of a ban on teaching LGBT issues in Scottish schools.
Sir Brian Souter, whose failed campaign to keep Section 28, a law banning teachers discussing gay rights in schools, is chairman of Stagecoach.
It is sponsoring the exhibition of the four shortlisted artists at the Turner Contemporary in Margate.
Stagecoach said its culture "values transparency, diversity, and respect".
The shortlist of artists was announced on Wednesday and their work tackles issues like oppression and marginalised communities.
Gay rights campaigner Peter Tatchell, told the Telegraph, he was "surprised and disappointed".
"Surely there must be other less tainted potential sponsors?," he added. "The arts is an LGBT-friendly profession and should not be colluding with companies whose leaders support homophobic discrimination."
By Will Gompertz, BBC arts editor
The bosses of Tate and Turner Contemporary must have known signing up Stagecoach as a sponsor of the 2019 Turner Prize might prove contentious.
If the fallout from the Sackler/Purdue Pharma controversy taught the sector anything, it was surely that a company and its bosses cannot be separated.
Sir Brian Souter's views are well known, he has expressed them in public on many occasions and they are readily available on his personal website.
Are they compatible with the values of the Tate, Turner Contemporary, the Turner Prize and this year's shortlisted artists?
After all, that is one of the basic tests an arts institution insists a sponsor must pass before it accepts a donation.
It is not unreasonable to assume in this case the implicit answer to that specific question is "yes" given none of those involved have said or done anything to suggest otherwise.
Speaking at a press conference, Tate Britain director Alex Farquharson said picking a sponsor "is very much a matter for the hosting venue".
Victoria Pomery, the director of Turner Contemporary, added "We have to take on board a whole range of issues when deciding our sponsorship.
"In this instance we decided that the role that the company plays in the area is very important."
In a statement, Stagecoach said it "does not tolerate discrimination or harassment of any kind based on disability, gender, gender reassignment, sexual orientation, religion, belief, age, nationality, race or ethnic origin".
It added: "We expect our employees to commit to doing the right thing, to respect other individuals at all times and treat them with dignity, and thoughtfulness, and we are committed to providing equal opportunities for all."
Last year's Turner Prize was won by artist Charlotte Prodger for her film on her experience of coming out as gay in rural Scotland.
This year's nominees are Lawrence Abu Hamdan, Helen Cammock, Oscar Murillo and Tai Shani.
The winner of the £40,000 prize will be announced on 3 December.