Alex McLeish has resigned as manager of Birmingham City.
Birmingham won the Carling Cup in February but went on to be relegated from the Premier League at the end of the past campaign.
The club's board had stated that McLeish's job was "safe" in the aftermath of their drop into the Championship but the Scot has now decided to leave of his own accord.
Aston Villa are already being suggested as a possible destination for McLeish.
"Birmingham City Football Club can confirm that Alex McLeish has resigned as manager," said a statement on the club's website.
"Acting chairman Peter Pannu received an e-mail resignation from McLeish earlier today [Sunday] while away on business in Amsterdam.
"No further comment will be made at this moment in time and updates will be released in due course."
McLeish took over at Birmingham in November 2007 and, after the St Andrew's club narrowly missed out on top-flight survival at the end of that season, he led the Midlands outfit back into the Premier League after the following campaign.
Birmingham stayed in the top flight for two seasons and beat Arsenal to win the Carling Cup in 2011 before they were again relegated after losing to Tottenham in a dramatic final day of action in the 2010/2011 season.
Although Birmingham's board were going to retain McLeish, they added the caveat that they expected him to achieve promotion once again but the 52-year-old has opted to move on.
How much substance there is in a possible switch to Birmingham's arch rivals Aston Villa remains to be seen but, if it did happen, it would cause plenty of controversy.
Villa failed in a bid to lure Roberto Martinez from Wigan as they continue to search for a successor to Gerard Houllier, who left at the start of July because of health issues.
"There is talk that Alex McLeish is now the number one target for Villa," said BBC Radio 5 live's Midlands football reporter Pat Murphy.
"This is fascinating because if McLeish is the number one target, Randy Lerner, the owner of Villa, is fully aware there will be a backlash from some Villa fans even in terms of approaching him.
"Lerner was concerned about a backlash in terms of possibly appointing Steve McClaren last week. I'm not sure what the fans would say about the manager of the Blue Noses going across the road to Aston Villa.
"I've heard from various sources that he is very much in the frame for Lerner. I'm not saying that it's a given but that's an intriguing name to throw into the mix, isn't it?"