It was high on the agenda of a conference of the Electrical Joint Industry Board, a forum for M&E unions and employers, that was due to be held in Harrogate next month. This has been postponed to allow the two groups to settle their differences.
AEEU national officer Paul Corby said: "There are many issues that we do not currently have consensus on, including the major projects agreement."
Corby would not say what the sticking point is, but it is understood that it concerns the definition of a "large project".
ECA director David Pollock played down the row, but said a number of issues needed to be agreed before the conference was held.
He said they were waiting for the results of a survey of workers' views of the deal. Delaying the conference, he said, would allow time to finish that study.
Last week it emerged that ESCA Services, the labour agency jointly owned by ECA and the AEEU, is to be sold because it is not making enough money.