Clive Ireson told Premier's News Hour proposals for just "one or two" were very messy.
It comes as Education Secretary Justine Greening confirmed the Government believed selection "can play a role" in the future of state education.
She raised expectations that Theresa May is poised to give the green light to a new generation of grammar schools in England.
But Ms Greening told the House of Commons there would be no return to the "simplistic" division of children into "winners and losers" at the age of eleven.
Mr Ireson said ministers had to "think about those children who aren't going to the grammar school" and what they would need.
Mrs May's announcement at a private meeting of Conservative MPs that she wanted a system "with an element of selection" has sparked an outcry among educationalists and opposition politicians.
The Government's social mobility tsar Alan Milburn warned that grammars could be "a social mobility disaster", while charity Teach First said: "Education experts are united that the evidence shows grammar schools harm social mobility."
Ms Greening was forced to the despatch box by an urgent question from her Labour shadow Angela Rayner, who warned MPs that an expansion of grammar schools would "entrench inequality and disadvantage".
The Education Secretary said she remained open-minded on the issue of selection and would announce the Government's policy in due course.
Mr Ireson said she should stop tinkering with policy: "They need to have a wholescale look, if they're thinking grammar schools are good, you don't just want them in one area".
