Education Secretary Michael Russell has revealed for the first time that he is prepared to revisit the terms of the 2001 teachers' agreement.
His admission provides momentum to cash-strapped councils' calls for changes to the landmark settlement which raised pay levels and enshrined the importance of non-class contact time.
Conservative education spokeswoman Liz Smith, who knew of two local authorities backing such a move, asked Mr Russell whether renegotiation might be necessary.
Mr Russell told the Scottish Parliament's education committee: "Any agreement that's 10 years old will probably need revisited, but I am not going to start the negotiations in public."
He stressed that any changes could only be made through discussion involving the Scottish Government, local authorities and education trade unions in the tripartite Scottish Negotiating Committee for Teachers (SNCT).
He would not be drawn on which aspects should be negotiated, but hinted that even education unions were beginning to accept a need for unpalatable changes to terms and conditions.
"I don't think any teachers' union leader has been anything other than realistic about the difficulties that exist in the public sector," he said.
Mr Russell described the target of 53,000 teachers in Scotland, set by the previous Labour and Liberal Democrat executive but also part of the SNP's 2007 manifesto, as "arbitrary".
Such targets had contributed to a "boom and bust in teacher supply in Scotland for as long as people have memories".
The fall in numbers in recent years was not all that it seemed, he suggested, as local authorities would have been "combing out" teachers whose roles did not involve spending much time with pupils.
Mr Russell remained as keen as ever on another Government target: "We are committed to class-size reductions now as wholeheartedly as we have ever been."
Few schools are believed to have reached the target of 18 pupils in P1-3, but he claimed that good progress was being made.
The Scottish Parliament this week agreed to a legal limit of 25 pupils in P1 classes - the previous cap was 30 - after an attempted block by Liz Smith was rejected by all other parties on the education committee.
Labour's Ken Macintosh probed the Education Secretary's enthusiasm for the limit of 20 pupils in S1 and S2 maths and English classes, asking whether it remained Government policy. It was still part of national guidance, Mr Russell replied, but it was more important to drive down class sizes in primary schools.
- Original headline: Russell will revisit terms of 2001 teachers' agreement