The Communication Workers Union is to affiliate to the Jeremy Corbyn-backing Momentum organisation.
The decision, taken in a vote by the CWU's National Executive Committee, comes after Momentum adopted a new constitution allowing the formal affiliation of trade unions.
Momentum emerged from the campaign to elect Mr Corbyn as Labour leader in 2015 and has been influential in encouraging supporters of his left-wing ideas to join and become active in the party.
Members of Momentum must also be party members and the group is seeking to become an official affiliate, but its activities are viewed with suspicion by some in Labour's centrist ranks.
CWU general secretary Dave Ward said: "The General Election showed the value of Momentum as part of the wider labour movement and it has established itself as a major political force in the UK.
"It has a key role to play in securing a transformative Labour government and we look forward to working with Momentum to help deliver this at the earliest opportunity.
"In the election Momentum helped reach out to new groups of voters and worked to mobilise, train and organise thousands of activists for Labour's campaign.
"The CWU will now be joining Momentum in its campaign to unseat prominent Conservative MPs and seeking to work with Momentum to reassert trade union values in society today."
The CWU has around 190,000 members in the communications industry and is the fifth largest union affiliated to Labour.
A CWU spokeswoman declined to say whether the union would make any financial contribution to Momentum as part of the affiliation arrangement.