Ed Miliband was booed by audience members during his address to the Trades Union Congress (TUC) annual conference in London. He was jeered after he said he had not changed his position that the public-sector strikes in June this year should not have taken place.
He said: "I fully understand why millions of decent public sector workers feel angry. But while negotiations were going on, I do believe it was a mistake for strikes to happen. I continue to believe that."
The strikes concerned the Government's public sector pension reforms. Critics of the reforms say that they will see public sector workers, such as teachers, nurses, and civil servants, work for more years and contribute more money to their pension, but receive less in return.
Miliband said: "The Tories have set about reform in completely the wrong way".
Labour would have done things differently: "We sat down and we negotiated. It was difficult but we got an agreement. That shows the way we should reform pensions in this country. It's not about change versus no change. It's about what kind of change, and how it's done.
"But what we need now is meaningful negotiation to prevent further confrontation over the autumn. Ministers need to show public sector workers - and the people who rely upon those services - that they are serious about finding a way forward."
Miliband urged the TUC members to continue negotiations with the Government, saying that industrial action should always be a last resort after all other avenues have failed.
He also warned unions that they have to think about their "relevance" going forward if they want to continue to have a place in Britain in the future.
Miliband said: "Unions can offer businesses the prospect of better management, better relationships. As you did during the recession."
But he added: "you will never have relevance for many workers in this country if you allow yourselves to be painted as the opponents of change".
Read more on the story (Telegraph)
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