Sajid Javid has said his resignation as Chancellor was necessary to uphold the "national interest" and sensible "checks and balances" in government.
He quit the cabinet in the prime minister's ministerial reshuffle after a row over his advisers.
In a personal statement in the Commons explaining his decision, he defended the Treasury's independence and said it must be able to "speak truth to power".
He also warned the government not to abandon its fiscal discipline.
