The MP for Kingswood said he was “increasingly concerned” over the former Chancellor’s “consistently changing position” on policy.
He has also urged other Tories to consider switching allegiance to the Foreign Secretary, adding “it is never too late to make the right decision”.
Such comments caused a source from Truss’ campaign to warn that the “floodgates could open”.
Mr Skidmore is known for his support of green policies and Net Zero, in comparison to most of Ms Truss’s backers who are more on the right of the party.
He wrote in The Telegraph: “The status quo cannot be an option. Initially, I had backed Rishi Sunak during the MPs’ stage of the contest.
“Yet over the past few weeks, I have grown increasingly concerned by his campaign’s consistently changing position, especially on the economy, to chase votes.
“I am convinced that we need a bolder, more positive approach to the UK’s future.
“Above all, we need a leader who will unite the party.
“Liz Truss has demonstrated that she has the leadership and personal ability to bring us all together.
“We cannot afford to be seen as a divided party, and I now believe that Liz is the best person to unite us and the country in meeting the challenges we face.”
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The Telegraph also reported that other defections could be possible, as a campaign source for Ms Truss has said the “floodgates could happen”.
A number of other Tory MPs who until recently had yet to declare have said they are backing the Foreign Secretary.
Damian Collins, Gordon Henderson, Henry Smith, James Davies, Mark Fletcher, Nigel Mills, Stephen MacPartland, Mary Robinson and Andrew Lewer co-signed a letter with Mr Skidmore that said: “We are all patriots and optimists, and we agree with Liz Truss’s assessment that our country’s best days are ahead of us.
“But this can only be true if we make it so, and that requires action.
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“We need to back our economy, support our constituents and our businesses, and drive the growth we need to get us back on track.
“That is why we are voting for Liz for Prime Minister.”
Ms Truss has been consistently polling ahead of Mr Sunak amongst Tory members, although figures from Mr Sunak’s campaign have questioned the validity of these surveys.
Former Tory leadership rivals Penny Mordaunt, Tom Tugendhat, Nadhim Zahawi and Sajid Javid have also come out in support of Liz Truss.