In his first public comments since the justices' decision on April 16, the prime minister said he was "really pleased" with the clarity that the court's ruling offers.
Starmer, who has previously said "trans women are women", was asked to repeat that statement but instead said: "I think the Supreme Court has answered that question."
Asked on Tuesday if that meant he did not believe a transwoman was a woman, he told ITV West Country: "A woman is an adult female, and the court has made that absolutely clear.
"I actually welcome the judgment because I think it gives real clarity. It allows those that have got to draw up guidance to be really clear about what that guidance should say.
"So I think it's important that we see the judgment for what it is. It's a welcome step forward.
"It's real clarity in an area where we did need clarity, I'm pleased it's come about.
"We need to move and make sure that we now ensure that all guidance is in the right place according to that judgment."
In a long-awaited judgment, the court confirmed the terms "woman" and "sex" in the 2010 Equality Act "refer to a biological woman and biological sex".
This means transgender women with a gender recognition certificate can be excluded from single-sex spaces if "proportionate".
Equalities Minister Bridget Phillipson said the ruling meant transgender women should use male toilets but suggested businesses should ensure "they have appropriate provision in place" for all customers.
That could mean the wider use of unisex facilities in cafes, pubs and restaurants in order to ensure "dignity" for all people.
Phillipson, who is set to update MPs on the Supreme Court's ruling, insisted there was unity within the government in welcoming the justices' decision.
The Mail on Sunday reported that ministers Chris Bryant and Angela Eagle were part of a WhatsApp group expressing concerns about the impact of the ruling and the Equality and Human Rights Council's response.
But Phillipson said: "I speak for the government on this matter and I can be crystal clear with you that we welcome the ruling.
"We will make sure that the EHRC work with us to bring forward guidance to address those areas where, through both the guidance and the statutory code of practice, providers have further clarity that is needed."
Phillipson also condemned the "completely unacceptable" scenes at protests following the Supreme Court ruling.
Seven statues around Parliament Square in Westminster were daubed with graffiti during a trans rights protest on Saturday.
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