Horner issued an immediate denial on Monday, stating: "I reject these claims entirely."
His defenders have claimed the accusations are part of a plot to oust him within the team amid an internal power struggle.
Horner's Spice Girl wife Geri Halliwell, 51, is said to be devastated by the bombshell allegations - but will firmly stand by her husband, who she married in 2015.
A source said: "[She is] not talking to anyone at the moment. Geri seems to be simply too upset to engage with anyone. Just lots of tears.
"She will stand by him and be at his side just as much as she always has been. She never misses a race and that will not change."
The world of Formula One was left in shock on Monday after it emerged that Horner had been placed under investigation by Red Bull Austria.
The parent company said that after being informed of “certain recent allegations” it had hired an “external, specialist lawyer” to probe the claims.
It has allegedly compiled a "dossier of incriminating material” regarding "inappropriate and controlling behaviour" towards one female staffer.
Red Bull is said to want the issue sorted out before the first race meeting of the new F1 season, which starts in Bahrain at the end of the month.
Neither Horner nor his accuser have been suspended and continue to work among 1,600 employees at the Austrian team’s HQ.
Christian Horner was pictured ahead of his hearing today over allegations of "inappropriate behaviour" towards a female member of staff.
The meeting - which will determine whether the F1 chief, 50, will keep his job as the team's head - was moved last-minute to a secret location.
