Ipswich chairman David Sheepshanks claims the desire to get back into the Premiership at the first attempt is the reason behind the decision to sack George Burley.
Town are 19th in the Division One table after winning just three of their first 10 games following their relegation from the Premiership last season.
Burley took over at Portman Road in December 1994 and guided the club into the Premiership in 1999-2000 before Ipswich secured a place in the UEFA Cup after finishing fifth in their first season back in the top flight.
But they struggled during the last campaign, which resulted in their relegation and a 3-0 loss at Grimsby on Tuesday was the final straw.
Sheepshanks said: "After the heady successes of our first year back in the Premiership, last season was a year of bitter disappointments, which culminated in our losing Premiership status. This was despite a huge investment in additional players for combined transfer fees approaching £15 million.
"The board still took the view that George Burley was the right man to lead us back to the Premiership, but this season, even though retaining a Premiership squad, performances have not improved.
"Given our single minded ambition to gain promotion this season the directors have decided that it is time to draw a line under a frustrating 12 months," Sheepshanks said.
Tony Mowbray is to take over on a caretaker basis and will be in charge for the home game against Sheffield Wednesday tomorrow.