The New York Times said Trump had fired Corey Lewandowski, citing two unnamed people. Lewandowski, who had managed Trump's campaign since it began last year, did not immediately respond to requests for comment.
A person working for the campaign told Reuters that some staffers were only learning of the departure through media reports.
"The Donald J. Trump Campaign for President, which has set a historic record in the Republican primary having received almost 14 million votes, has today announced that Corey Lewandowski will no longer be working with the campaign," Hope Hicks, the campaign spokeswoman, said in a statement.
Lewandowski's departure comes as Trump, the presumptive Republican nominee, has faced setbacks in the past two weeks. His renewed calls to ban Muslims from entering the United States have drawn heavy criticism from Republican politicians in Washington and prompted corporate sponsors like Apple (AAPL.O) and JPMorgan Chase (JPM.N) to withhold funding from the party's July nominating convention in Cleveland. Recent opinion polls show at least half of all voters saying they approve of the idea.
Lewandowski, a former New Hampshire field director for a conservative advocacy group, has been with Trump since the wealthy New York developer began his unconventional White House bid a year ago. Lewandowski had spent the past several months in a power struggle with the more traditional Republican strategists that Trump had hired more recently to try to reshape his campaign operation.
Most prominent among them is Paul Manafort, an adviser to several Republican presidential campaigns, including those of Ronald Reagan and George H. W. Bush. Manafort and others had been pressuring Trump to hire more staff and tone down his fiery public persona, but their early efforts were rebuffed, according to several people familiar with the exchanges. Manafort did not respond to a request for comment, and Trump has often dismissed reports that Manafort and Lewandowski did not work well together.
Lewandowski, who traveled to nearly every Trump rally, insisted in interviews that Trump's unconventional campaign, with its shoestring budget, light staffing and miniscule fundraising apparatus, did not need to be changed because Trump's massive victories in many primary elections early this year proved it was successful.
Known for his brusque manner, Lewandowski was accused of manhandling a female reporter in Florida during the primary campaign, but the charge against him by a local sheriff was later dropped.
He is still scheduled to attend the Cleveland convention as the chairman of New Hampshire's delegation.