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Nick Boles has resigned as Skills Minister just hours before Theresa May is due to succeed David Cameron as prime minister.
In a statement on Facebook, he said: "As David Cameron prepares to visit HM the Queen and tender his resignation I want to add my voice to those praising his remarkable service as Prime Minister and leader of the Conservative Party".
"It was a huge privilege to be part of his Government. I want to thank him for offering me the opportunity to serve my country. I believe that this is the right time for me to return to the back benches."
He offered his full support to the incoming prime minister "as she strives to make Britain a place that works for everyone, not just a privileged few. It is a noble ambition and I wish her well."
Mr Boles was a key ally of Mr Cameron's during his bid to modernise the party in opposition between 2005 and 2010.
He campaigned on behalf of the Remain campaigner during the EU referendum and has notably supported Mr Cameron's intiatives such as same-sex marriage which were not supported by the grassroots membership.
He said: "The modernisation of the Conservative Party is a process, not an event, with different phases – and different emphases as it unfolds.
"Changing the social attitudes of the party so we learned how to treat women and gay people and people of different races and faiths with equal respect was a vital step, first pioneered by Theresa May as Party Chairman."
While acting as Michael Gove's campaign manager during the Conservative leadership contest, Mr Boles texted potentially wavering Tories who had previously backed Ms May to ask them to back the Justice Secretary to block Andrea Leadsom from the run-off, saying it was "in the national interest".
In a text message to MPs, published by the Sun, he asked "I am seriously frightened about the risk of allowing Andrea Leadsom onto the membership ballot.
"Are we really confident that the membership won't vote for a fresh face who shares their attitudes about much of modern life? Like they did with IDS".
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Ms May won 199 of the votes cast by the party's MPs but the Energy Minister and Brexiteer came second on 84 after the second ballot. Mr Gove came third on 46 - two fewer than he had received in the first round.
Mr Boles was also said he would "sleep easily at night" with Ms May as Prime Minister and that Mr Gove didn't "mind spending two months taking a good thrashing from Theresa".
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