Steve Lomas was sacked as Millwall manager on Thursday just hours after his struggling Championship side were thrashed 4-0 by Watford.
Former Manchester City and West Ham midfielder Lomas was dismissed just six months into his reign at The Den following a miserable run of results in the second tier.
The Lions are languishing just three points above the relegation zone after their third successive defeat and Lomas, who was never accepted by many Millwall fans due to his time as captain of their bitter rivals West Ham, was told he was sacked in a meeting with chief executive Andy Ambler soon after full-time at Watford.
"We are very disappointed that it hasn't worked out for Steve," Ambler told Millwall's website.
"He has been thoroughly professional in his approach to the job during his short period with the club and remains a very promising young manager.
"However, the board feels that it is in the best interests of Millwall that we make the change at this time and the search for a successor to Steve begins immediately.
"We thank Steve for his efforts and wish him the very best for the future."
Lomas joined Millwall after making his name at Scottish Premiership side St Johnstone, who he led into the Europa League last season.
But the former Northern Ireland international failed to replicate that success in south London.
He lasted just 22 league matches with Millwall and oversaw only five victories, with reports claiming he had lost the confidence of several first-team players.
First-team assistant coach Neil Harris and academy director Scott Fitzgerald, both former Millwall players, will take temporary charge of the team for the trip to Doncaster on Sunday.