Three major brewers - Shepherd Neame, Marston's and Fuller's - have been excluded from full membership of the Society of Independent Brewers (SIBA).
The decision is a result of new membership rules agreed unanimously by SIBA members at its AGM yesterday, including an upper threshold of 200,000 hectolitres for full membership.
However the three brewers excluded will remain as associate members, a new status agreed upon by SIBA.
Associate members will be able to engage with SIBA, but have no voting rights.
SIBA chief executive Julian Grocock said: "This is without doubt a milestone moment for SIBA, in that we have, for the first time in our 29-year history, a clear and documented statement on what it means to be a SIBA member.
"With this issue resolved, we are now in better shape to really start making progress on our stated ambition to help smaller brewers bring their beers to market."
The new threshold aligns SIBA membership with the European definition of a 'smaller brewer'.
A new package of measures around membership also includes a requirement for all new members to sign up to the new SIBA Charter, and to serve 12 months as provisional members before being granted full membership.
Grocock added: "The upper limit is not about excluding larger brewers from engaging from SIBA, indeed I am confident that now we have clearly defined their role, more of them will be keen to join us as associates.
"We have many goals in common and I look forward to working together with them and other industry partners in pursuit of a better future for British beer."