Image caption Paul Kenny said Ed Miliband's speech had "far reaching consequences"

Plans to change the way union members donate to Labour could "end rather than mend" relations, says the leader of the UK's third largest union, the GMB.

Paul Kenny told the BBC that Labour leader Ed Miliband's "bold move" was "as close as you can get" to ending the link between Labour and the unions.

He predicted a fall in Labour funding.

Mr Miliband wants union members to have to actively opt in to join Labour, rather than being automatically affiliated as part of union membership.

Mr Kenny said the move could see a 90% drop in the number of GMB members affiliating to the party.

The union says its affiliation fees paid to Labour could drop from around £2m a year to less than £1m as a result of the changes.

Mr Kenny, its general secretary, said the union would decide whether to ballot its members on whether they still want to give money to Labour in September - meaning the funding cut could happen as early as next year.

Under the current system members are given the chance to vote on a union's political fund every 10 years.

Media playback is unsupported on your device Media caption Ed Miliband: "I do not want any individual to be paying money to the Labour Party in affiliation fees unless they have deliberately chosen to do so"

But Mr Miliband said on Tuesday he would end the automatic "affiliation" fee paid to Labour by three million union members and people should not pay Labour any fees "unless they have deliberately chosen to do so".

The changes would have "massive financial implications" for the party, he acknowledged, but could raise its membership from the current 200,000 to a "far higher number".

The announcement came after Unite, one of the party's biggest donors, was accused of signing up its members to Labour in Falkirk - some without their knowledge - in an effort to get its preferred candidate selected to fight the next election.

"This is an absolute watershed moment - the changes they are about to do will fundamentally change the party's relationship with trade unions and their members," Mr Kenny said.

'Insulting'

He told BBC Radio 4's Today programme that collective affiliation and individual membership were two "entirely different things".

Media playback is unsupported on your device Media caption GMB's Paul Kenny: "If they wanted to, they could join the Labour Party like that now"

The union would be "lucky" if 10% of the 400,000 members who are currently affiliated to Labour on a collective basis would choose to sign up as individual members, he said.

"We have been trying to encourage our members to join Labour for a very long time - through magazines, meetings and our conference. I have to say we have not been knocked down in a rush.

"The jump will be from currently affiliating the union to argue for people on political policies with the Labour Party to individual membership of the party. Lots of people agree with the first but I am not sure that many people agree with the second.

"If they wanted to, they could join the Labour Party like that now."

Mr Kenny criticised the language he said had been used by some Labour politicians in recent weeks describing it as "disappointing and insulting" and said he would not be surprised if some members wanted to disaffiliate from Labour entirely.

Gerry Morrissey, the general secretary of media and entertainment union Bectu, which is also affiliated to Labour said he believed Mr Miliband's proposals were "totally unnecessary", as members could choose to opt out of the 3p a week payment on their membership forms.

If the proposal went through, he said, "then the Labour Party will lose money".

"I don't know what percentage of ours would opt in but if you looked at normal ballot procedures etc, I wouldn't be expecting much more than 20-25% of people to be opting in," he told BBC Radio 4's World at One. His party has donated more than £50,000 to Labour since the general election.

Members of both Unite and the GMB recently voted to maintain their political funds.

Unite general secretary Len McCluskey has said the union's political levy - worth £8m a year to Labour - would "stay as it is" and he is happy to discuss the proposals with Labour as the current relationship between the party and the unions is not satisfactory.

Mr Miliband's initiative has been welcomed by senior figures in the party, including Tony Blair, who said he wished he had done it when he was party leader.