Derrick King, the Bridgend chairman, said: "Following the exit of Bridgend from the Swalec Cup and in the light of a number of defeats in the National League, John Phillips has tendered his resignation.
"After discussing the matter with Mr Phillips the directors have very reluctantly accepted the resignation and he will be leaving the service of the club immediately."
The team now face two vital matches on successive Saturdays - Ebbw Vale tomorrow followed by Newport - and King said that he believed they were "possibly the two most crucial games in the history of the club".
The coach, Steve Brown, the backs coach, Alan Donovan, and the former Welsh and British Lion full-back JPR Williams will take temporary charge of the team.
"It really is a nightmare - I never thought we would go into the Christmas period with this sort of situation," said King.
Leicester Tigers have been able to select an all-international pack of forwards for today's trip to table-topping Saracens.
The return of the No 8 Eric Miller from Ireland duties makes it possible, with Lewis Moody reverting to the bench for the Allied Dunbar Premiership match.
The stand-off Joel Stransky returns to the Leicester side after his rib injury, with Fiji's Waisale Serevi switching to scrum-half. But the England centre Will Greenwood is ruled out with a shoulder injury and the 23-year- old James Overend steps in for his first league appearance of the season.