Sir Michael said the number of poor pupils admitted to state grammars – currently around three per cent – was 'nonsense' and suggested that campaigners pushing for an expansion were misplaced.
But his comments sparked outrage among grammar school supporters who insisted they remained hugely popular with parents.
In an interview with The Observer, Sir Michael said grammar schools were not the answer to driving up standards, following the rejection last week of plans for a new grammar in Sevenoaks, Kent.
He said: "Grammar schools are stuffed full of middle-class kids. A tiny percentage are on free school meals: three per cent. That is a nonsense.
"Anyone who thinks grammar schools are going to increase social mobility needs to look at those figures. I don't think they work.
"The fact of the matter is that there will be calls for a return to the grammar school system. Well, look what is happening at the moment."
But Robert McCartney, chairman of the National Grammar Schools Association, said: "I think it is astonishing. The real question here is not about grammars versus state comprehensives, it is about whether we have enough good schools.
"The fact is, we have 10 to 15 applications for every grammar school place and the reason so many aspirational parents are determined to get their children in is because they recognise they are good schools. In reality we need more, not fewer, grammar schools to meet this demand."
He added: "It surprises me that someone in his position should mount an attack on that one section of the state education system that is providing top class results. He should be preserving the best and worrying about the rest."
Plans for the new grammar in Sevenoaks – supposedly an 'annexe' of an existing school – were rejected by the Department for Education last week because they fell foul of legislation banning the opening of wholly new state grammars in England. It would have created 1,300 new places in the Kent town.