Just 12% of Unite members would sign up as full Labour Party members, new polling from Lord Ashcroft claims.
The former Tory party deputy chairman said just 30% of Unite members would opt in to funding Labour if reforms to the way the political levy is operated went through.
Less than half (49%) said they would vote Labour if an election were held tomorrow, compared to 23% Conservative, 7% Liberal Democrat and 12% Ukip. At the 2010 election 40% voted Labour, 28% Conservative and 20% Lib Dem.
Labour leader Ed Miliband has proposed ending the automatic opt-in of union members which is currently a major source of funding to his party.
Lord Ashcroft said: "(Unite general secretary) Len McCluskey rightly observes that whether individual trade unionists will rally to Labour will depend on whether Miliband gives them 'reasons to want to be associated' with the party. This is largely about policy. But the policies he himself advocates seem unlikely to have the desired effect. McCluskey is quite right that his members are not queuing up to join Labour. And if Miliband takes his advice, nor will they."
The poll interviewed 712 Unite members from a total membership of 1.42 million. Questions were posed online between July 10 and 17. Other findings of the poll suggested 30% of Unite members already opted out of funding the Labour Party, while a third did not know whether they did or not. Most Unite members, 57%, supported the idea of an opt-in system.
The union's membership was divided over whether Unite had been right to give nearly £12 million to Labour since the general election, with 46% disagreeing but 43% backing the donations. Almost half, 49%, said future large donations would be wrong.
Unite members were more likely to think the Labour Party did a bad job of representing ordinary working people (47%) than think it did a good job (42%). And a large majority of 86% of members backed the Government's policy of a £26,000 a year benefit cap.
Mr Miliband said: "I think this poll is proving that we are right to want change, because there are people who want to be part of our party, want to be active members of our party, want to be affiliated to our party. I want to give that that choice and I want to make it a real choice."
If 12% of individuals currently affiliated to Labour through their unions signed up to become full members, that would double the size of the party membership, he pointed out.