A former British intelligence officer who is now a director of a private security-and-investigations firm has been identified as the author of the dossier of unverified allegations about President-elect Donald Trump’s activities and connections in Russia, people familiar with the matter say.

Christopher Steele, a director of London-based Orbis Business Intelligence Ltd., prepared the dossier, the people said. The document, which an official close to the matter said was prepared under contract to both Republican and Democratic adversaries of Mr. Trump, alleges the Kremlin colluded with Mr. Trump’s presidential campaign and claims that Russian officials have compromising evidence of Mr. Trump’s behavior that could be used to blackmail him.

Mr. Trump has dismissed the contents as false and Russia has denied the claims.

Mr. Steele, 52 years old, is one of two directors of the firm, along with Christopher Burrows, 58.

Mr. Burrows, reached at his home outside London on Wednesday, said he wouldn’t “confirm or deny” that Orbis had produced the report. A neighbor of Mr. Steele’s said Mr. Steele had said he would be away for a few days. In previous weeks, Mr. Steele has declined repeated requests for interviews through an intermediary, who said the subject was “too hot.”