Hull have begun the search for the manager to save their season after Mike Phelan left the club.
The Tigers announced the exit on Tuesday night of 54-year-old Phelan, who was only appointed as boss on a permanent basis in October after an interim stint in charge.
Monday's 3-1 loss at West Brom left Hull deep in Premier League relegation trouble, with 13 points from 20 games, and they sank to the foot of the table on Tuesday after Swansea's win at Crystal Palace.
A Hull statement read: "The club can announce this evening that it has parted company with head coach Mike Phelan.
"We would like to thank Mike for his efforts both as assistant manager and head coach over the last two years.
"The search for a replacement has begun and an announcement will be made in due course."
The club did not say whether Phelan was sacked or resigned.
Hull will be keen to get their new manager quickly in place, and bookmakers made former Birmingham boss Gary Rowett an early favourite, ahead of Alan Pardew, Wales' Chris Coleman, former England boss Roy Hodgson, Harry Redknapp and Ryan Giggs.
Former Manchester United midfielder Phelan stepped in after previous manager Steve Bruce left the club shortly before the start of the season. Bruce had only months earlier led Hull into the top flight.
The Tigers won their first two league matches of the campaign, against last year's champions Leicester and Swansea, but have been unable to build on that strong start. Their last win came on November 6 against Southampton.
They sit three points behind 17th-placed Palace, the last safe team, but have a far inferior goal difference to the Eagles and cannot afford to let points drain away as they did under Phelan, who was hampered by meagre resources.
Reacting to the departure, former England striker Gary Lineker wrote on Twitter: "Mike Phelan fired by Hull. Not sure what they expect given the squad he inherited."
The incoming manager will have the benefit of an open transfer window in which to do business, although drawing players to a relegation battle may demand strong powers of persuasion.
Their next game comes in the FA Cup third round on Saturday, coincidentally against a Swansea side who were rejuvenated after a change of boss at Palace, having brought in Paul Clement to replace Bob Bradley.