Melissa Phillips has been sacked as Brighton manager with the club 10th in the Women's Super League.
Phillips, 36, became Brighton's third boss of 2022-23 when she joined the club in April and guided the Seagulls to safety last season.
They are currently six points clear of the WSL relegation place.
Technical director David Weir said it was "not a decision which has been taken lightly, but we feel it is vital for the progress we want to see".
BBC Sport understands Phillips was informed late on Thursday afternoon and the players were told shortly afterwards.
The Seagulls have reached the quarter-finals of the Women's League Cup and the fifth round of the FA Cup.
Weir said: "We have invested heavily in the women's squad and infrastructure going into this season, and results and performances have not been at the level we had expected, given that investment."
Mikey Harris moves from the men's academy to take charge as interim manager, beginning with the WSL match at Manchester United on Sunday (12:00 GMT).
Managing director Zoe Johnson added: "The immediate priority is upcoming WSL, Conti Cup and FA Cup matches against Manchester United this Sunday, and Aston Villa and Wolves the following week; while also beginning the search for a permanent head coach."
Analysis - 'Announcement comes as a shock to many'
BBC Sport reporter Emma Sanders
I was at the Brighton training ground last week and there was no indication Phillips' job was at risk.
The former London City Lionesses boss is well respected within the game and arrived at the club when they were a point adrift at the bottom of the table with seven matches left - albeit with two games in hand.
Brighton did spend heavily in the summer transfer window and there has been investment in facilities, staff and infrastructure in recent seasons.
But there appeared to be an understanding that Phillips, who has had just nine months in the role, would need time to work with the squad and the players recruited - including 11 summer signings.
Melina Loeck, who joined in January, told me on Friday you could "see the improvements throughout the first half of the season" which included a win over Manchester City, as well as a draw with Manchester United.
Expectations were high in pre-season and while Brighton have definitely not lived up to those, they are still six points clear of relegation and this announcement comes as a shock to many.