Teachers are prepared to launch a legal challenge over "sneaking" plans to expand selective education.
Kevin Courtney, the National Union of Teachers (NUT) general secretary, said "a few" schools have been identified which it believes are setting up "grammar streams" - selective education in comprehensive schools based on the results of a test.
Teaching staff at the union's annual conference in Cardiff on Saturday are expected to vote on taking possible legal action against individual schools.
The vote comes in the same week that Education Secretary Justine Greening said selection in new 21st century grammar schools would increase the options for young people "to truly help make the most of their talents".
Mr Courtney said: "The Conservatives did not include this in their 2015 manifesto.
"If the Government has legislated not to have new grammar schools, there shouldn't be ways of sneaking around it.
"That's what we think schools are doing at the moment."
He said the NUT has written to Ms Greening complaining about selection within academy trusts, but had not received a reply.
He told reporters: "The idea that you can have grammar schools open to all, we think is an oxymoron.
"What does that mean to say grammar schools can be open to all? That sounds like comprehensive schools to me.
"If they (the Government) really wanted to have a selective system that was unbiased across social classes, then they know how to do that. You could arrange children into groups based on parental income, administer a test and admit 20% from each group into a grammar. You could have pupils from every social group.
"The trouble with that, from the Conservative point of view, would be that would mean 80% of middle class kids would not be admitted to grammar schools.
"The Government is looking at other ways of selective schooling - having off-shoots, and having selective schools within a multi-academy trust and that they will say that's like a stream in a secondary school.
"We think that would be illegal and open to legal challenge."
Chancellor Philip Hammond previously set out plans for £500 million for additional spending on schools in England, with £320 million for 140 new free schools - including new grammar schools promised by Theresa May.
Labour introduced new laws in 1997 banning grammar schools from being set up.
A so-called "priority motion" put to delegates at the weekend will ask whether the NUT executive should "investigate possible legal routes to challenge the expansion of selective education".
Mr Courtney said: "We know the Government has no manifesto backing for grammar schools.
"We think that they are aware that they would have a very difficult parliamentary passage," he said, citing a perceived lack of support in the House of Lords, and the amount of legislative time being taken up with Britain's withdrawal from the European Union.