Nicky Morgan has backed Michael Gove in the Tory leadership race.
Mr Gove made a surprise announcement on Thursday morning that he is putting himself forward to succeed David Cameron, saying he does not believe Boris Johnson can "provide the leadership or build the team for the task ahead".
In a statement, the Education Secretary said she has "concluded unequivocally" that Mr Gove has the "skill and credibility to put together the right team to renegotiate our exit from and future relationship with Europe, and explain the final terms to the British people."
Michael Gove has become the first of the Conservative leadership candidates to publish his tax returns during the campaign.
Theresa May has secured the backing of Philip Hammond in her bid to succeed David Cameron ahead of the first round of voting on Tuesday.
A poll in the Sun on Sunday suggests the home secretary is able to count on the backing of six out of 10 Conservative MPs.