The CEO of Donald Trump’s social media empire was branded a “proverbial loser” whom the former president “would have fired on the Apprentice” by a trading firm owned by the billionaire Republican donor Ken Griffin on Friday.

In an extraordinary statement, Citadel Securities accused Devin Nunes, chief executive of Trump Media & Technology Group (TMTG), of trying to deflect blame for the company’s recent stock market woes. TMTG hit back claiming Citadel was “world famous for screwing over” small investors.

Hours earlier, it emerged that Nunes – a former Republican congressman who resigned two years ago to run TMTG – had written to the head of New York’s Nasdaq stock exchange to raise “potential market manipulation” of the group’s stock.

Shares in TMTG, which owns Trump’s Truth Social social network, have come under sustained pressure since enjoying a stunning market debut last month. They have rallied in recent days, however, in volatile trading.

In a letter on Thursday to Nasdaq, Nunes suggested so-called “naked” short-selling was behind the market turbulence that has plagued TMTG – and named several large trading firms, including Citadel Securities, which he claimed were behind most of the trading.

Short selling is a way of betting against a public company. An investor borrows a stock, and then sells it on; should the stock then drop, the investor then buys it back and pockets the difference.

So-called “naked” short-selling is selling a stock without first borrowing it, or determining if it can be borrowed. It is generally illegal in the US, as Nunes noted in his letter.

“Devin Nunes is the proverbial loser who tries to blame ‘naked short-selling’ for his falling stock price,” a spokesperson for Citadel Securities said. “Nunes is exactly the type of person Donald Trump would have fired on the Apprentice. If he worked for Citadel Securities, we would fire him, as ability and integrity are at the center of everything we do.”

Trump Media said: “Citadel Securities, a corporate behemoth that has been fined and censured for an incredibly wide range of offenses including issues related to naked short-selling, and is world famous for screwing over everyday retail investors at the behest of other corporations, is the last company on earth that should lecture anyone on ‘integrity’.”

Shares in TMTG rose 9.6% on Friday.