Hart quits QPR after five games
QPR said in a statement on their website that Hart, sacked by Premier League Portsmouth in November, had decided to leave with immediate effect.
Assistant manager Mick Harford, who was also a caretaker manager at QPR in 2007, will take charge of this weekend's match at Blackpool.
Hart was appointed on December 17 after Jim Magilton was sacked following media reports of a bust-up with Hungarian midfielder Akos Buzsaky.
Media reports on Friday suggested Hart had been undermined by a disagreement with Moroccan midfielder Adel Taarabt, who is on loan from Tottenham Hotspur.
The west London club have had six full time managers since flamboyant Italian Flavio Briatore, the former Renault Formula One boss, took over in late 2007.