The body that represents Catholic schools has accused the Northern Ireland Office (NIO) of excluding it from a conference focused on inclusivity.
The Council for Catholic Maintained Schools (CCMS) described its absence as ‘alarming’ and the Controlled Schools Support Council (CSSC) said it was also not invited to the event, which was titled A More Confident and Inclusive Northern Ireland.
Neither the Integrated Education Fund nor the Northern Ireland Council for Integrated Education were invited either.
The three-day forum began on Wednesday and was attended by Secretary of State Brandon Lewis, who said the UK government was set to do more to promote integrated education.
In a letter seen by BBC News NI, the CCMS said it was “ironic” that they were not chosen to attend.
"The programme asks, 'what can be done to ensure the views of all young people, from all communities, are given the opportunity and tools to engage?'," their letter said.
"Given that the broad family of Catholic schools represents one of the largest providers of education in Northern Ireland, the notable absence of participation and involvement with the Catholic sector is alarming.
"Indeed, it is rather ironic that a conference with a focus on building inclusion is actually exclusionary in its approach.”
"The conference programme asks the question: 'How to work together to build a confident and inclusive Northern Ireland for all?' their letter continued.
"Coupled with the exclusion of all Catholic schools and the bodies who represent them at the landmark event despite the track record of excellence and success within Catholic education, we ask the question to the Northern Ireland Office: 'Does the NIO recognise the contribution of Catholic schools to supporting our young people to reach their full potential and where does the NIO see Catholic schools within its future vision?'"
There are approximately 450 Catholic maintained schools in Northern Ireland, which are attended by roughly 150,000 pupils.
Controlled schools are those formerly managed mainly by the Protestant churches, which were transferred to the state in the 20th century and are now under the management of the schools’ Board of Governor
The controlled sector is the largest education sector in Northern Ireland. There are 552 controlled schools in Northern Ireland, representing 49% of all schools.
The chief of its umbrella body, the CSSC, further penned a letter to principals of controlled schools, noting that they "raised concerns that any conference which includes a discussion on the future of education would be held without the voice of 49% of the schools in Northern Ireland being present”.
"We were given assurances that the NIO wish to engage with the sector,” Mark Baker continued.
A UK government spokesperson said: "The Wilton Park conference has brought together a variety of academic, public policy, and third sector voices to consider how to work together to achieve a more confident and inclusive Northern Ireland.
"A broad range of factors are under consideration, including skills, education, the economy and social integration.
"The NIO will continue to engage across all sections of society in its work going forward, including the Council for Catholic Maintained Schools and the Controlled Schools Support Council, and bodies in the education sector."
Both the CCMS and CSS, along with the three other main representative bodies in education, have also previously expressed opposition to an Integrated Education Bill for Northern Ireland,, which passed through the Assembly in March despite the lack of a functioning government.
The bill, from Alliance MLA Kellie Armstrong, seeks to compel the Department of Education to increase the number of integrated school places and set minimum targets for the number of children being educated in integrated schools.