Grammar schools are a form of "state sponsored segregation", the Lib Dem education spokesperson has said.
Layla Moran told Lib Dem activists that grammar schools did ''nothing to drive social mobility'' and that they were a ''waste of money''.
The former teacher also reiterated the party's call for the abolition of Ofsted, an overhaul of league tables and primary school exams and a pay rise for teachers.
Ms Moran, speaking yesterday at the Lib Dem conference in Brighton, said: "It says everything that only 3 per cent of children on free school meals go to grammars.
"How, when we know they do nothing to drive social mobility, does the government push them to expand? What a waste of money.
"How, when we know families spend thousands of pounds on private tutoring to pass the 11+ can the government claim that they are accessible to all?"
It comes after education secretary Damian Hinds told Tes there was evidence that selective schools can improve social mobility.
In May, he had announced details of a £200 million fund to allow existing grammar schools to expand.
Ms Moran added: "So what I do want to do - what I would like to see us do as a party, hand in hand with the communities that this affects - is take on the harmful, antiquated tradition of the 11+ exams and indeed all barriers to entry in the state system.
"I'm especially talking about those areas where families have no choice whatsoever about taking the exams.
"Where creaming off the middle classes is demonstrably stifling social mobility for those worse off.
"In those areas, grammar schools are in effect state sponsored segregation.
"It is wrong and it needs to stop."
Ms Moran also accused the Tories of allowing "educational purging", saying: "It beggars belief that we insist on continuing to publish league tables which cultivate a damaging system where schools compete so ferociously that they forget their true purpose: to improve education for all.
"Low performing pupils from poorer families were scrubbed from the roster to make the school look better, following a policy encouraged by the Government.
"Theresa May is literally scrubbing out those children's futures. I call this educational purging. And it is a disgrace."