The government has been accused of a "shamelessly partisan attack" on Labour funding after changes to union subscriptions were included in the Queen's Speech.
The Conservative election manifesto pledged legislation to ensure trade unions use a "transparent opt-in process for union subscriptions".
But when the Trade Unions Bill was published today, the words "for the political fund element" were added.
With little explanation of what is being proposed, unions now assume that members will have to opt-in to paying an annual amount to Labour, rather than opting out as at present.
There is speculation that the measure is bound to hit union funds to Labour.
At present, unions collect millions of pounds from their members every year for political funds, including those not affiliated to Labour.
A Unite spokesman said: "The Tories are taking Britain back to the 1920s with this shamelessly partisan attack on the funding of the opposition party.
"Political funds are already subject to approval being given in regular ballots by unions. Tory hedge fund and multimillionaire donors will face no similar restrictions, leaving boards free to write hefty cheques backing the Tory Party.
Paul Kenny, GMB general-secretary, told the Press Association: "This is just the same old Tories who are anti-trade union and anti-working people having a voice.
"It's one rule for the Tory slush fund, hedge funds and another for trade union members.
"This will not deter or silence the voices of millions of working people who have already given their approval for political funds through democratic ballots governed by statute.
"It will bring state funding for political parties a step nearer. It is not sustainable to allow the elite and companies unfettered and unlimited rights to fund the Tory Party while shackling the bodies that have funded the political opposition to them for more than a century."
A Labour Party spokesperson said: "On the union opt-in, we support measures to ensure increased transparency and have already introduced reforms to Labour Party processes to reflect this.
"Any proposals from the government must be done in a fair non-partisan way and which doesn't rig things in favour of the Tory Party."
How the process works
As mentioned earlier, unions collect millions from their members for political funds, including those not affiliated to Labour.
Details from 26 unions in the 2012/13 year showed that around 4.8 million members contributed to a political fund, with fewer than 850,000 not contributing.
A total of 13 unions are affiliated to Labour, including the biggest in the UK such as Unite, Unison and the GMB.
Teachers, civil servants, railway workers and firefighters are in unions not affiliated to Labour, but they still pay into a political fund for campaigning.
Members of unions affiliated to Labour typically pay a few pounds a year, raising millions for the party.
A Downing Street spokesman said: "No-one is suggesting that you should be able to opt in or opt out of paying your union subscription. If you are members of the union you pay your subscription.
"The only debate is about the political levy and that is a debate that has been going on for decades and so I think it's pretty well understood.
"At the moment you are supposed to be able to opt out. There is already concern that at the moment that isn't being spelled out very clearly to new members. Our approach is that there should be a conscious decision of opting in.
"An opt-in already exists in Northern Ireland. So it is not an alien concept.
"There has been a long debate about it, we put it clearly in our manifesto, we were elected on that manifesto.
"Clearly some union leaders may not be very happy about it but that doesn't mean it's not right."
Skills minister Nick Boles said: "The Conservative Party manifesto made a clear pledge to reform the trade union rules on union subscriptions. In Northern Ireland, trade unions already ask for their members if they wish to opt-in to paying the political levy.
"Even Ed Miliband backed the principle of opting in for trade unions affiliating members to the Labour Party. Political donations should be voluntary and there is a clear case for greater transparency."
What do you think? Is the government meddling in union affairs, or is this fairer on union members, who up until now were automatically signed up to donating to Labour?
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