Ruth Davidson told an audience in Washington DC the re-evaluation was part of a “massive, massive shock” induced across Europe by the arrival of the unpredictable President.
Referring to the UK-US relationship, she said: “We’re going to want to make sure that any deals that are done he’s going to honour. We have to be sure of that.”
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She also said the Trump White House was “chaotic” because of “a lack of professionalism and moral seriousness”, with staff who were one step away from “white supremacist bloggers”.
The comments jar with the friendly tone struck by Theresa May when she visited Mr Trump to cement the UK’s “special relationship” with the US in January.
The Prime Minister also spoke to Mr Trump by phone on Valentine’s Day, and said she “looks forward” to his state visit this summer.
Ms Davidson does not attend cabinet meetings, but has a standing invite to attend Mrs May’s “political cabinets”, when political strategy is discussed rather than government business.
Speaking at the Women in the World event in the US capital on Wednesday, Ms Davidson said caution was required with a President whose early actions should “worry us all”.
She said: “At the moment, from the UK, we’ve always seen America as being a very strong, a reliable ally and now, even after only 26 days or however long the tenure has been so far in Pennsylvania Avenue, we are beginning to reassess how reliable an ally the United States is.
“And that’s a huge change in Europe. That’s a massive, massive shock.”
Discussing Mr Trump’s attacks on the media, she added: “We have to be very careful about popular nationalists, populist strongmen, part-demagogues, using a lot of their political capital at the beginning of their journey to de-legitimise public scrutiny, which is what demonising the media does.
“I think that if we learn our lessons from history, there is a very strategic reason why you do that. I think that should worry us all.”
Before he was elected, Ms Davidson said Mr Trump's "impetuosity could be dangerous not just for America but for the world".
After Mr Trump introduced a botched ban on immigrants from seven Muslim-majority countries last month, she also said his state visit to the UK should be put on hold until it was repealed.
Four in five were supportive, a freedom of information request revealed.
A Tory MP has called for a no confidence vote in Mr Bercow, as the Speaker, parliament’s highest officer, must remain politically impartial at all times.
A Downing Street source said the UK wanted to have the "strongest relationship possible" with the US.