Darren Ferguson has resigned as Peterborough United manager in the wake of Saturday's 1-0 defeat to Championship relegation rivals Derby County.
Posh had identified February as being a key month in their battle for survival, with games against fellow strugglers Reading and Derby.
However, after failing to beat Reading on Wednesday night, their hopes took a further dent on Saturday as Louie Sibley's last-gasp goal secured a precious win for Derby at Pride Park.
The result means Peterborough are now second from bottom in the Championship, five points adrift of safety.
The club confirmed on Sunday evening that Ferguson had tended his resignation to the owners after a phone call with co-owner Darragh MacAnthony.
MacAnthony, and the club's other joint owners, had previously insisted that they wouldn't sack Ferguson under any circumstances and wanted him to continue.
"Myself and my two partners regard Darren as one of the greatest managers of this football club, most certainly in the modern era," co-owner Darragh MacAnthony said.
"The success we have enjoyed with him at the helm has been unrivalled and he will always be described as a legend by myself and my partners."
Ferguson, who returned to Posh for a third spell as manager in 2019, led the club to automatic promotion from League One last season.
That was his fourth promotion with Peterborough, further cementing his status as a club legend.
MacAnthony added: “I want to personally thank him for all his hard work on behalf of the club, particularly in his third spell when he dealt with the unquestionable frustration at the season being cut-short due to Covid-19 and then responding by securing promotion the following season against all the odds.
“The financial constraints he had to work under due to the impact of no supporters inside the stadium was something that nobody could have foreseen, and he dealt with that in such an admirable and methodical way. He created a working environment that the players enjoyed despite the restrictions and he deserves enormous credit for that and the success we enjoyed.”
In a statement released to the League Managers Association on Sunday evening, Ferguson said that he hopes the club will still be able to survive relegation.
"After working tirelessly and winning promotion to the Championship, I genuinely hope that the club can find a way to remain there," he said.
"Thank you to the players and staff, who were committed to the task in hand. Particularly Liz, Kieran, Barry and Bob.
"I would also like to express my respect for Darragh MacAnthony who has shown endless support and commitment to Peterborough United throughout the years. I’m sure the fans will continue to get behind him and the new management, no matter what the position of the club is."
Analysis - ITV News Anglia sports reporter Andy Ward
The fact that Ferguson has resigned tells you everything you need to know about how much he cares about Peterborough United.
Despite huge pressure from some fans, co-owner Darragh MacAnthony had repeatedly insisted that he wouldn't sack Ferguson and there was no indication that his stance would have changed.
However, there's no doubt that the failure to beat Reading and Derby County is a hammer blow to Posh's hopes of staying up, perhaps a fatal one, and Ferguson has put the club before himself in the hope that the Great Escape can still be achieved.
The fact is that Peterborough still have 15 games to save themselves, and a fresh voice might just be what's needed to get them over the line.
However, many Posh fans will undoubtedly be devastated that it hasn't worked out for Ferguson in the Championship - a man who has given his all for the club in three hugely successful spells in charge.
It would be a huge surprise if Ferguson ever returns to the club for a fourth stint at some point in the future, but if this is to be the end, MacAnthony is right to say that Ferguson will rightly go down as one of the club's greatest ever managers - a legacy to be truly proud of.