The move spells the end for UCATT as an independent union and has been forced on the organisation by a long-running financial crisis.
UCATT said: “The union made the decision in order to preserve its existing structures and to maximise the representation of construction workers in all sectors.”
The move will raise fears among contractors as they are now forced to negotiate with a larger and more powerful union.
One industrial relations expert said: “UCATT has been on its backside financially for years and the contractors were well aware of its problems.
“Dealing with Unite is a different proposition and they have historically been much more aggressive in their tactics.”
UCATT will now enter into full negotiations with Unite the union.
Once negotiations are complete a vote on a transfer of engagements of all UCATT members will take place.
UCATT said the vote followed a “full, frank and passionate debate” about the union’s future.
Brian Rye, Acting General Secretary of UCATT, said: “UCATT delegates listened to the arguments both in favour of remaining independent or a merger or transfer to Unite and decided that the best option for existing members and for all construction workers was a link up to Unite.”
Len McCluskey, Unite general secretary, said: “We welcome this chance to work with UCATT colleagues on building a powerful, united union for construction workers where we can combine our might and expertise to get the best deal for this workforce.”