Billionaire investor Robert Mercer, who was Donald Trump’s biggest single donor during the presidential election, is to sell his stake in the controversial rightwing news site Breitbart.

The 71-year-old also plans to step down as co-chief executive of hedge fund Renaissance Technologies from 1 January, according to a letter he sent to investors.

Mercer and his daughter, Rebekah, are Republican mega-donors and patrons of Steve Bannon, the chairman of Breitbart News and former White House chief strategist known for his populist views.

In a letter to colleagues at the hedge fund, Mercer acknowledged that he has come under scrutiny and said “for personal reasons” he has decided to sell his stake in Breitbart News to his daughters, Bloomberg reported.

“The press has also intimated that my politics marches in lockstep with Steve Bannon’s,” Mercer wrote. “I have great respect for Mr Bannon, and from time to time I do discuss politics with him. However, I make my own decisions with respect to whom I support politically. Those decisions do not always align with Mr Bannon’s.”

Turning to notorious former Breitbart provocateur Milo Yiannopoulos, Mercer reportedly added: “I supported Milo Yiannopoulos in the hope and expectation that his expression of views contrary to the social mainstream and his spotlighting of the hypocrisy of those who would close down free speech in the name of political correctness would promote the type of open debate and freedom of thought that is being throttled on many American college campuses today.

“But in my opinion, actions of and statements by Mr Yiannopoulos have caused pain and divisiveness undermining the open and productive discourse that I had hoped to facilitate. I was mistaken to have supported him, and for several weeks have been in the process of severing all ties with him.”

He added: “For personal reasons, I have also decided to sell my stake in Breitbart News to my daughters.”

Mercer’s politics have put him at odds with those of other top figures at Renaissance. Jim Simons, who remains chairman of the Renaissance board, was one of Hillary Clinton’s top financial backers during last year’s election.

Mercer was a leading financial backer of Cambridge Analytica, a data mining company that worked with the Trump campaign. The WikiLeaks founder Julian Assange said last month that Cambridge Analytica had approached it during the election but did not give further details.

Trump was a guest last December at Mercer’s annual holiday costume party at Owl’s Nest, his Long Island estate. The president’s adviser Kellyanne Conway tweeted: “Honoring the ultimate hero at the Mercer ‘Heroes and Villains’ party on Long Island. Crowd thrilled w/ surprise!”

Honoring the ultimate hero at the Mercer "Heroes and Villians" party on Long Island. Crowd thrilled w/ surprise! pic.twitter.com/VQAyaKvs6i — Kellyanne Conway (@KellyannePolls) December 4, 2016

Mercer will leave the board of Renaissance but remain at the company, which manages more than $50bn in assets, as a member of the technical staff focusing on research work.

The news took many observers by surprise.

“It’s really extraordinary,” Joseph Lowndes, associate professor of political science at the University of Oregon, told the Guardian.

It was Mercer who introduced Bannon to Trump and who brought him on board at Breitbart, Lowndes added. “Bannon owes his political fortunes to Mercer. Mercer is also the connective tissue between Bannon and Breitbart and Trump. He is a less understood and more important figure in the American political landscape than people realise.”

Lowndes suggested that the sale of Mercer’s stake in Breitbart to his three daughters, including key donor Rebekah, is little more than a cosmetic change.

“My sense is that Rebekah is as politically passionate as her father and is oriented precisely as he is,” he said. “Unless she also has some political change of heart, I would imagine it would have very little impact.”

Lowndes added: “It may simply be that the reclusive Mercer was comfortable supporting politically controversial figures and projects on the far right as long as this work was largely unscrutinised.”

Dan Cassino, a political science professor at Fairleigh Dickinson University and expert on conservative media, agreed: “I’m not sure we have any daylight between Robert and Rebekah Mercer but it’s interesting that Robert is trying to distance himself from Steve Bannon’s politics. Rebekah hasn’t been as associated with Bannon.”

He added: “I don’t think it makes much difference to Breitbart but it might make a difference to how much Bannon is able to direct to candidates personally.”