Vicky Pryce, 60, emerged from East Sutton Park Prison near Maidstone, Kent, via a back exit.
The mother-of-five was driven away with her lawyer Robert Brown, pursued by press photographers who been camped outside the Category D open jail for women and young offenders.
The ex-wife of former Cabinet minister Chris Huhne's has served around a quarter of an eight-month sentence for taking speeding points for him.
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The coup shows a split in the cabinet between ministers who want to stop a no-deal Brexit, and those who want to prevent a referendum.
Speculation that the PM will be ousted from Downing Street has reached fever pitch.
Newly-released papers show a young David Cameron condemning ‘Euro red tape’ while working as a researcher for the Conservative Party in 1988