The Labour party will now "automatically" support all strikes taken by trade unions, the new shadow chancellor said last night.
John McDonnell told a conference fringe meeting that Labour would have "absolute solidarity" with all actions taken by the trade union movement.
He said the party needed to become a "resistance movement".
"And that means absolute solidarity. The view now is straightforward and I tell you this: If there is industrial action taking place then we should automatically now, automatically come alongside our brothers and sisters in the trade unions and support them."
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He said the former party leadership had been too hesitant to publicly support strikes.
"Time and time again in the hierarchy of the Labour party there has been a hesitancy about this. I think we have to break that hesitancy and give people the confidence and courage and determination to start standing up," he told the Assembly Against Austerity.
"My role, whether it is in parliament, or on the picket line, is to support workers in struggle.
"That is what the role of Labour MPs is going to be in the future."
McDonnell is chair of the Public and Commercial Services union (PCS) group in parliament. He was joined last night by PCS general secretary Mark Serwotka, who praised McDonnell for giving "hope and inspiration to millions of people."
He suggested that PCS and other trade unions would now seek to launch co-ordinated strike action across the public sector, with the support of the Labour leadership.
"There should be united, co-ordinated strikes… And the thing about the strike that may well come our way, is unlike in 2011 [the] Labour leadership when asked to support the strikers shuffled embarrassedly and looked at their shoes, we now have a leader of the opposition and a shadow chancellor who will support these strikes."
Serwotka, who was barred from voting in the Labour leadership contest, added that Corbyn's election and McDonnell's appointment meant the Labour leadership was now "well to the left of the TUC".