The Government is facing further unrest from teachers with a second union rejecting its £27 per hour offer on supervision and substitution work.
The TUI voted by 55:45 to reject the offer and the union's negotiators will now have to look for improvements from the Department of Education.
The vote is a significant boost for hardline elements in the ASTI who have been pressing for some time for a common approach between themselves and the TUI.
The general secretary of the TUI, Mr Jim Dorney, last night said his union would seek immediate talks with the Department to discuss an improved offer.
TUI sources said its members had rejected a specific offer, but had not voted to withdraw from supervision. This means school closures will not result from the vote - at least not in the short term.
"We have managed to keep the TUI out of industrial trouble so far, so we will simply have to go in and see if the Government can improve on this offer," said one senior TUI source.
The Minister for Education, Dr Woods, said he was disappointed at the result, but he and his officials would be happy to meet the TUI "to find a resolution to this dispute".
The turnout of 64 per cent was higher than the recent 50 per cent turnout among ASTI members on the supervision issue.
The union is likely to press for the offer to be made pensionable, but this could upset other public sector unions who may regard that as a general pay increase.
The primary teachers' union, the INTO, is also balloting its members on the £27 offer, but the result will not be known for weeks. While it is regarded as the most moderate of the three unions, the Programme for Prosperity and Fairness was barely passed by its members last year. However, its executive is recommending acceptance of the £27 offer.
It is unclear whether the TUI will work jointly with the ASTI to secure a better offer. The ASTI leadership's attempts to get improvements were rebuffed this week by Department officials.
The main reason most TUI members voted no was: concerns over the non-pensionability of the offer; optimism the offer could be improved and general discontent with the Government's handling of the teachers' pay issue.
Last night, school managers reacted with dismay at the result. Mr Michael Moriarty, general secretary of the Irish Vocational Education Association (IVEA), said managers would do everything in their power to keep schools open in the months ahead.