Monitor’s board has appointed its vice chair, Chris Mellor, as interim chair following the departure of Bill Moyes.
Chief executive duties are being shared between Mr Mellor, the former chief executive of Anglian Water Group, chief operating officer Stephen Hay and director of strategy Adrian Masters.
An interim chief executive appointment is expected to be made quickly, Monitor said.
Mr Mellor, who has been on Monitor’s board since 2004, left Anglian Water Group in 2003 following criticism of some of his management decisions.
He is currently non-executive chair of Water Services in Northern Ireland and was a non-executive director of Addenbrooke’s Trust from 1994 to 1998.
Mr Moyes’ term as executive chair ended at the end of January and the government decided not to extend it.
The government failed to hire a permanent replacement chair after an appointments process late last year, and the post was readvertised on 10 January.