The Labour grassroots group Momentum is supporting shadow business secretary Rebecca Long-Bailey in the race to replace Jeremy Corbyn as party leader.
Some 70.4% of members who responded backed a proposal to support Mrs Long-Bailey, with 52.1% opting to back Angela Rayner for deputy leader.
A Momentum spokesperson said it would be “mobilising thousands” to campaign for its chosen candidates.
Mrs Long-Bailey and Ms Rayner were the only names on the online ballot.
This followed their selection as preferred candidates by Momentum’s steering group.
After the announcement of the results, Mrs Long-Bailey, an ally of Mr Corbyn, said she was “proud and beyond grateful” to be backed by the group.
She said she would