'He will take the whole government with him,' said the Northern Ireland minister.
Steve Baker has stated that he supports Rishi Sunak to be the next PM as Boris Johnson will "definitely explode".
The minister of Northern Ireland and Johnson's ally said that "Boris would be a guaranteed disaster" because of the ongoing investigation into Partygate.
He suggested that the former PM should do something "big and statesmanlike", such as becoming chairman of the Conservative Party, rather than return to Downing Street.
Mr Baker told Sky's Sophy Ridge on Sunday: "I have been a huge Boris Johnson fan many times but this is not the time for Boris style... he doesn't have a strict adherence to boring rules."
He also said: “He will definitely explode, bringing down the entire government with him. We can't do that anymore."
Cabinet Minister Penny Mordaunt is the only candidate to declare that she is officially running for PM.
Mr Sunak is the only one to receive the more than 100 support required by his fellow Tory MPs.
Johnson flew back from the Caribbean on Saturday and held talks with Sunak, who reportedly over whether they can enter the leadership race with tickets together.