All 12 top-flight clubs met at Hampden on Tuesday to discuss plans for league reconstruction amid other radical reforms, including an earlier start to the season, a winter break and the re-introduction of play-offs.
Afterwards SPL chairman Ralph Topping and chief executive Neil Doncaster claimed to be optimistic the plan to have an SPL 1 and 2 with 10 teams in each division would be ratified at the next meeting on January 17 but Thompson told STV: "I know there's certainly another three clubs who are against the current proposals."
An 11-1 vote is required to push the proposal through.
Thompson added: "It's up to us to stand firm in what we believe in. If we believe it's not right, then we won't vote for it.
"Or rather than not vote for it, which means voting against change, abstain, which is as good as a no vote anyway. In the current format I don't think it's going to happen, unless something dramatic changes within the proposals."
A strategic review group featuring Topping, Doncaster and representatives from six clubs - Hibernian, St Mirren, Motherwell, Celtic, Rangers and Aberdeen - formulated the proposals following research by Doncaster, who believes a 10-team top flight is the only proposal which can deliver on a financial basis.
However, like Dundee United, Hearts are believed to remain unconvinced by the plans and Thompson called for further discussion.
He said: "Don't get me wrong, there's a lot of good things that the Strategic Review Group have come up with, but I'm certainly not going to vote for it in its current format.
"I think there's a lot of good things in it, a lot of good things that could happen without changing everything, but it seems to be take it all or nothing. I'm not against change, but it's all about the right change."