The body representing Catholic schools has accused the Northern Ireland Office (NIO) of excluding it from an event on Northern Ireland's future.
The NIO is behind a conference called A More Confident and Inclusive Northern Ireland.
In a speech at it, Northern Ireland Secretary Brandon Lewis said the UK government was set to do more to promote integrated education.
The umbrella body for controlled schools also said it was not invited.
The Council for Catholic Maintained Schools (CCMS) and Controlled Schools Support Council (CSSC) described their absence as "concerning" and "alarming".
