Michael Gove has vowed to “keep shtum” over who he will vote for in the Tory leadership contest.
The environment secretary said both Boris Johnson and Jeremy Hunt “would do a great job for this country”.
He insisted he wanted to make sure he could give “100% support” to whoever becomes the next PM.
Mr Gove initially backed Mr Johnson in the 2016 Tory leadership race. Later, though, he decided he was not up to the job and chose to run against him.
At the time, Mr Gove said he decided to withdraw his support because Mr Johnson was “not capable” of leading the party or the country.
Shortly afterwards, Mr Johnson pulled out of the leadership contest, which was eventually won by Theresa May.
One Johnson ally said at the time Mr Gove’s actions were “one of the biggest acts of treachery I have ever seen”.
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Speaking in London on Tuesday, Mr Gove said Mr Johnson had “championed the environment” in all his previous roles.
“As foreign secretary, he’s been a powerful and persuasive voice on safeguarding wildlife from exploitation, further protecting our oceans and fighting climate change.”
He also praised the work of Mr Johnson’s rival, Jeremy Hunt, whom he said had worked to “safeguard oceans and help to put more international development assistance towards environmental projects”.
“I know they both would be great prime ministers and I want to affirm today that we can trust them both to do the right thing on every critical issue facing us and of course most critically on the environment.”
Conservative Party members are currently voting for their choice of leader via postal ballot.
The winner and next prime minister will be announced on 23 July.
Asked to say who he wanted to win, Mr Gove replied: “I won’t say which of the two candidates I am going to vote for.
“I want to give whoever wins 100% support so for that reason I’ll keep shtum.”
Tory leadership race: Michael Gove ‘keeping shtum’ over choice}