Speaking in City Hall today, Khan rejected Prime Minister Theresa May's claims that reintroducing the schools, along with the associated testing for students, would raise standards.
Instead, he warned the programme would inevitably lead to segregation.
Only four per cent of schools in London are classified as grammar schools, but the capital was recorded as the best performing part of England in GCSE results for the seventh consecutive year.
“There is no evidence that grammar schools raise standards for the poorest students and we know that the best performing international education systems are not selective,” Khan said.
He added: “Selection leads to segregation and I believe that increasing the number of grammar schools will only make it harder to ensure that every child is able to fulfil their full potential, and it would guarantee that some children are left behind. Unlike the government, I'm not prepared to give up on providing a place at an excellent school for every child in London
“Opening new grammar schools was the wrong policy before, and is still the wrong policy now. I call on the new government to drop these plans.”
It comes after May last week made a major speech in which she said she wanted the UK to become “the great meritocracy”.
"I want to see children from ordinary, working class families given the chances their richer contemporaries take for granted," she said.