Hart, who took over as caretaker manager following Tony Adams' departure in February, has left Fratton Park after guiding the south coast club to just two victories from 13 Premier League games this season.
"Portsmouth Football Club can confirm that Paul Hart has been relieved of his duties as first-team manager," read a club statement.
"The decision has been made reluctantly by the board and is based on results that leave the club at the bottom of the Premier League."
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The former Nottingham Forest and Barnsley manager declined an offer to remain at Fratton Park in a technical director role.
Pompey have now pledged to make a swift appointment with recently returned director of football and ex-Chelsea boss Avram Grant the frontrunner for the job.
"Paul has worked under very difficult circumstances with the financial restrictions the club has faced since he took over," chief executive Peter Storrie told the club's official website.
"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.
"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."
Source: DSG
