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Miss Patel's departure follows Sir Michael Fallon's decision to quit as defence secretary last week, over accusations of misconduct against women.
The government was already being propped up on the slenderest of margins by the DUP, and question marks are still hanging over the futures of further frontbenchers in Boris Johnson and Damian Green.
Her elevation to the Cabinet maintains the delicate balance of Brexiteers and Remainers in Cabinet, while ensuring that the same number of women have seat on the top table.
EU leaders are said to be preparing for the fall of Mrs May's government within weeks after scandal and divisions threatened to erode its slender grip on power.
The Portsmouth North MP was a prominent campaigner in the referendum, notably clashing with David Cameron on live TV over whether Turkey was set to join the EU.
She also has a colourful back story, having served as a magician's assistant, trained as a Royal Navy reservist and topped the 2011 list of sexiest female MPs.
The PM condemned her comments as 'absolutely wrong' and the clash was a rare 'blue on blue' row in the campaign.
She has served as Armed Forces minister and had been tipped to take over from Sir Michael Fallon as Defence Secretary after he was forced to quit over sex sleaze claims.
She became one of the leading figures in the Leave campaign and clashed with her boss Mr Cameron live in television over her claims Britain would not be able to stop Turkey joining the EU.
Her deputy Damian Green is still being investigated over sex harassment allegations, while Foreign Secretary Boris Johnson is facing demands to resign over loose comments that might double the jail term for a British mother in Iran.
Ms Mordaunt's appointment to the job maintains the careful balance of Remainers and Brexiteers around the Cabinet table and Tory MPs warmly welcomed the move.
She admitted making a joke speech about animal welfare so she could say the word 'c**k' after a dare from her friends in the Navy.
The MP for Portsmouth North has lived in the large naval town since she was two year-old and her family are steeped in its military tradition.
Her father was in the Paras before becoming a teacher and she is a Royal Navy Reservist.