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 “deliver on the trust Momentum members have placed in me, with a plan to win the next general election and transform our country for the future”.
The shadow business secretary is up against four other MPs in the contest to succeed Mr Corbyn – Lisa Nandy, Jess Phillips, Sir Keir Starmer and Emily Thornberry.
The Labour ballot opens on 21 February and closes on 2 April. The results of both the leader and deputy leader elections will be announced at a special conference on 4 April in London.
To make it onto the final ballot candidates need the support of 5% of local parties or at least three affiliates – two of which must be unions – by 14 February.
Sir Keir has secured the backing of the union Unison, while the National Union of Mineworkers has thrown its weight behind Ms Nandy.
On Saturday, Momentum said a meeting of its steering group had unanimously agreed that Mrs Long-Bailey was “the only viable candidate” to continue Mr Corbyn’s “socialist agenda”.
The two propositions – that Momentum should support Mrs Long-Bailey as leader and Ms Rayner as deputy – were put to members, 7,395 of whom participated.
Momentum is believed to have about 40,000 members.
The decision not to ballot members on the other candidates has been criticised by former Momentum national co-ordinator Laura Parker.
“Although I am pleased Momentum’s governing body accepted the principle of balloting its members on the leadership, I’m sorry they seem to have decided in advance what the answer is,” she tweeted.
“Members should be able to choose from all leader and deputy leader candidates.”