New chancellor Nadhim Zahawi has urged unity after his predecessor, the health secretary, and several junior ministers walked out.
But the prime minister has been hit by six further resignations, taking the total to 16 in the past day.
It comes as he prepares for PMQs later and a grilling by senior MPs.
Mr Johnson's premiership has been plunged into crisis following the dramatic resignations of Chancellor Rishi Sunak and Health Secretary Sajid Javid.
They quit within minutes of each other on Tuesday following a row over Mr Johnson's decision to appoint Chris Pincher deputy chief whip earlier this year.
Their departures triggered a wave of resignations from more junior roles that has continued on Wednesday.
In six further departures ahead of PMQs, education ministers Will Quince and Robin Walker, Justice Minister Victoria Atkins, Treasury minister John Glen, and ministerial aides Laura Trott and Felicity Buchan have all walked out.
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