The NEU says it is looking into the legality of a letter sent by government to teachers claiming the current pay offer will be automatically applied.
Deputy Elaine Millar wrote to teachers on Friday (12 January) saying the current multi-year pay offer will automatically apply unless they opt-out.
Those who take the deal will not be able to take part in any industrial action over pay until at least 2027.
Ministers have been branded 'nasty and cowardly' by opposition politicians for insisting individual teachers reply by email if they are declining the pay offer.
The National Education Union is advising its members not to respond to the letter while it gets legal advice, even though the government says that anyone who doesn't reply to opt out by 1 February will be taken as having accepted.
The States Employment Board is offering an 8% consolidated pay uplift in 2024, along with a one-off non-consolidated payment of £1,000.
Teachers who accept the offer will also get inflation-linked increases, or 2% (whichever is greater), in 2025 and 2026.
An NEU spokesperson says the union remains in dispute.
"Teacher recruitment and retention remains at crisis level and the Government’s continued refusal to maintain the value of teacher pay will simply make this worse. The NEU remains in dispute with the Government regardless of whether they decide to pay of the rejected offer and to whom.
We will be consulting with our members and national executive shortly over the next steps in our campaign including consideration of further industrial action."
The NASUWT union narrowly voted in favour of accepting the deal in December.
The government have been asked for comment.