The chief foreign affairs correspondent of The Wall Street Journal was fired on Wednesday after evidence surfaced that he had become entangled in the business dealings of an Iranian-born aviation titan who was one of his key sources.

The correspondent, Jay Solomon, was a highly regarded veteran of The Journal and Washington diplomatic circles, and his dismissal came as a shock to the paper’s newsroom.

Shortly after the firing was announced, The Associated Press published a report detailing Mr. Solomon’s communications with Farhad Azima, an Iranian-born businessman, about a venture that at times involved aerial espionage in Iran and the sale of defense equipment to the United Arab Emirates.

It was unclear if Mr. Solomon ever formally entered into a commercial arrangement or received compensation tied to Mr. Azima, according to the A.P.

In a statement to The A.P., Mr. Solomon conceded “mistakes in my reporting” and apologized to his colleagues.

“I never entered into any business with Farhad Azima, nor did I intend to,” he said. “But I understand why the emails and the conversations I had with Mr. Azima may look like I was involved in some seriously troubling activities.”

Mr. Azima has had a colorful career in the aviation and defense industries, including a cameo in the Iran-contra scandal of the 1980s, when he was named as the owner of an aircraft used to clandestinely ferry American military equipment to the Iranian armed forces.

Journalists for The A.P. were investigating Mr. Azima when they obtained a huge trove of electronic documents late last year; the documents revealed correspondence between Mr. Solomon and Mr. Azima about a potential business arrangement. The A.P. notified The Journal of its findings and asked about Mr. Solomon’s involvement.

“We are dismayed by the actions and poor judgment of Jay Solomon,” said Steve Severinghaus, The Journal’s communications director. “The allegations raised by this reporting are serious. While our own investigation continues, we have concluded that Mr. Solomon violated his ethical obligations as a reporter, as well as our standards.”

The statement added, “He has not been forthcoming with us about his actions or his reporting practices, and he has forfeited our trust.”

In The Journal’s Washington bureau, the mood on Wednesday was one of astonishment. Although The A.P. was close to publishing its report, many of Mr. Solomon’s colleagues were unaware of his firing until shortly after 3 p.m., when the bureau chief, Paul Beckett, called an abrupt all-hands meeting and announced it.

Colleagues described Mr. Solomon, who is based in Washington, as a serious and enterprising journalist who was seen as a star at The Journal from a young age.

He had worked there for about two decades, including overseas assignments in Asia and Africa, according to a biography on his personal website. Recently, he was the paper’s lead reporter on the negotiations over nuclear arms between Iran and the Obama administration, traveling to the Middle East to meet sources and dig up scoops.

His firing was the latest in a spate of unwelcome news for The Journal. Layoffs and buyouts have agitated the newsroom, and the newspaper’s editor has been forced to defend its coverage of the Trump administration, which some critics have said was too soft. The paper has also lost senior writers and editors, including the Washington correspondents Carol E. Lee and Damian Paletta, as well as the deputy editor in chief Rebecca Blumenstein and the columnist Bret Stephens, who both joined The New York Times.