General secretary Steve Sinnott told the union’s annual conference in Harrogate: “The NUT’s exclusion from education policy matters is an outrage. We have been working hard to end this exclusion.
“Today I can tell conference we have initiated legal proceedings against the secretary of state.”
The union has been excluded from major talks since it refused to sign up to a national deal on reforming teacher workloads.
Sinnott has written a formal letter to Johnson and is awaiting a response before the union issues formal proceedings.
The NUT successfully sued former education secretary David Blunkett over performance-related pay.