Red Bull has initiated an independent investigation into allegations related to an event attended by Horner, Toto Wolff, and Zak Brown.
German publication F1-Insider.com first published the allegation, with the investigation subsequently confirmed in a statement to De Telegraaf.
“After the company was made aware of certain recent allegations, an independent investigation was launched,” that began.
“This process, which is already ongoing, is being carried out by an external, specialised lawyer.
“The company takes these matters extremely seriously and the investigation will be completed as soon as practical. It would not be appropriate to comment further at this time.”
Horner has rejected the allegations, reportedly made by a former staff member.
Horner has been in charge at Red Bull Racing since it joined the sport in 2005 and has led the team to seven drivers’ championships and six constructors’ crowns.
Earlier this year, he was named among the King’s New Year Honours, which saw him elevated to Commander of the Order of the British Empire (CBE).
He was appointed an Officer of the British Empire in 2013 after four successive championship wins with Sebastian Vettel.