Mr Walker was a technology teacher at the secondary school
A teacher who posted comments on a right-wing website during a lesson could be struck off.
BNP member Adam Walker, 39, left his post at Houghton Kepier College, near Sunderland, last year after admitting contributing to the forum in work time.
He made critical comments about Islam, asylum seekers and immigrants.
It has now emerged he will face a General Teaching Council hearing and could become the first teacher banned for religious intolerance.
The technology teacher's case is being fought by the trade union Solidarity, which is closely linked to the BNP and of which he is president.
'Freedom of expression'
Solidarity general secretary Patrick Harrington said Mr Walker had admitted he was wrong to use his laptop to contribute to the forum during work time.
He said: "That was a contractual matter between him and his employer and when he resigned, that should have been the end of the matter.
"We feel this raises human rights issues. He has a right, like every other citizen, to freedom of expression and association.
"He criticised Islam in his posting, but people criticise religions all the time."
Mr Harrington said the union would take the case to the High Court if they lose the GTC hearing, which will be held in due course.
Mr Walker's brother Mark, also a BNP member and technology teacher, was suspended by Sunnydale College in Shildon, County Durham, in March last year.
His supporters claim he was suspended for looking at the BNP website on a school computer.
The school denies it has acted because of his political affiliation.