The grassroots are expected to back a motion calling on Nick Clegg to "rule out" any expansion of the policy.
Simon Hughes, the LibDem deputy leader, said that party members had already been "fairly robust" in their opposition.
Liberal Democrat members are set to protest against regional pay proposals at their annual conference this weekend – in a move party chiefs predicted could kill the plans.
The grassroots are expected to back a motion calling on Nick Clegg to "rule out" any expansion of the policy.
Simon Hughes, the LibDem deputy leader, said that party members had already been "fairly robust" in their opposition.
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