The departure of the distance-learning institution, which had been a member of the group since it was established in 2007, means that 23 universities are now part of the Alliance.
Rajay Naik, director of government and external affairs at The Open University, said that although the university was ending its “formal association” with the group, he was “confident our ties will remain strong”.
“The OU is proud of its distinctiveness and we will continue to work with our partners across the sector and beyond to strengthen the UK’s position as a world leader in higher education,” he said.
Steve West, University Alliance chair and vice-chancellor of the University of the West of England, added: “The Open University is a great institution doing exciting things, particularly online, but it is a different kind of institution from the rest of the sector.
“We look forward to continuing to work with them on delivering wider participation and innovative solutions for higher education.”
Despite The Open University’s move, the Alliance remains the second-strongest mission group – not including representative bodies like GuildHE and Universities UK - in terms of membership numbers. The Russell Group has the most members with 24 institutions on its books.
A statement from The Open University said there were “no plans at the moment” to join another mission group.