A spokesman for parent company Dentsu Aegis Network UK & Ireland declined to comment on the reason for Muscant’s sudden exit from the company.
Muscant (pictured, above), who has been chief executive since September 2016, headed a 600-strong team that was spread across four cities, Manchester, Edinburgh, Leeds and Newcastle.
The group comprises regional operations for agency brands including Carat, Vizeum and iProspect.
The 40-year-old started as a graduate trainee at MediaVest, an independent Manchester agency, in 2000.
He became part of the management team that sold to Aegis, which was subsequently bought by Dentsu.
The company spokesman said: "Dentsu Aegis Network UK & Ireland can confirm that Elliot Muscant has stepped down from his role as chief executive of Dentsu Aegis Network North with immediate effect.
"With a strong leadership team in place across our North operations, alongside the support of Dave Lucas, chairman, Dentsu Aegis Network North, Stef Calcraft, executive chairman, Dentsu Aegis Network UK & Ireland, and the wider executive team, it will remain business-as-usual for our teams and our clients.
"An announcement on our succession plan will be made in due course."
Dentsu Aegis Network North is an unusually large regional operation in the London-dominated UK agency marketplace.
Other agency groups such as WPP don’t have a large, standalone regional operation.