http://www.teamtalk.com/football/story/0,16368,2483_5716416,00.html
Former Nottingham Forest and Barnsley boss Hart has left Fratton Park with Pompey at the foot of the Premier League table, after taking just seven points from 13 matches.
They were beaten 1-0 at Stoke on Sunday and are four points adrift of safety, ahead of this weekend's clash with Manchester United.
Former Chelsea manager Grant joined Portsmouth as director of football last month but it is understood he is not a candidate to succeed Hart as manager.
Coaches Paul Groves and Ian Woan have taken temporary charge of the first team.
Portsmouth said in a statement that Hart was "relieved of his duties", and stressed the decision was taken "reluctantly" and on the basis of recent results.
The club said Hart turned down the role of technical director, with responsibility for players aged 18 to 21.
Portsmouth chief executive Peter Storrie told the club website: "Paul has worked under very difficult circumstances with the financial restrictions the club has faced since he took over.
"However, the board feels that the team should have accrued more points to date and that we need a new man in charge to ensure Premier League survival.
"The board would like to thank Paul for his work not only in keeping the club in the Premier League last season but also for his time in charge of youth development.
"Paul is a man of great dignity and we hoped he would stay and help us develop younger players on the fringes of the first team.
We are genuinely sad to see him leave. Everyone at the club wishes him well for the future."
Former Nottingham Forest and Barnsley boss Hart has left Fratton Park with Pompey at the foot of the Premier League table, after taking just seven points from 13 matches.
They were beaten 1-0 at Stoke on Sunday and are four points adrift of safety, ahead of this weekend's clash with Manchester United.
Former Chelsea manager Grant joined Portsmouth as director of football last month but it is understood he is not a candidate to succeed Hart as manager.
Coaches Paul Groves and Ian Woan have taken temporary charge of the first team.
Portsmouth said in a statement that Hart was "relieved of his duties", and stressed the decision was taken "reluctantly" and on the basis of recent results.
The club said Hart turned down the role of technical director, with responsibility for players aged 18 to 21.
Portsmouth chief executive Peter Storrie told the club website: "Paul has worked under very difficult circumstances with the financial restrictions the club has faced since he took over.
"However, the board feels that the team should have accrued more points to date and that we need a new man in charge to ensure Premier League survival.
"The board would like to thank Paul for his work not only in keeping the club in the Premier League last season but also for his time in charge of youth development.
"Paul is a man of great dignity and we hoped he would stay and help us develop younger players on the fringes of the first team.
We are genuinely sad to see him leave. Everyone at the club wishes him well for the future."