A Labour leadership contest is the ideal time for trade unions to push their case for more power and rights to strike, Tony Benn has argued.
The veteran Labour politician called on unionists pressing for a repeal of Margaret Thatcher's anti-trade union laws to write to all the potential candidates to demand their views - and publish these to union members before any leadership vote.
"I'm given up protesting - it shows you've lost. We should be demanding," Mr Benn told a fringe meeting at the TUC in Brighton this evening.
Bob Crow, general secretary of the RMT train drivers union, repeated the point, telling the packed meeting: "The likes of Brown, Blair, Hain and Johnson are going to be swarming around here in a few months - ask them what they are going to do."
Union leaders have been flexing their muscles recently in recognition of their financial influence - unions made up 75 per cent of Labour donations in the last three months - and in the knowledge of their significant vote in a leadership contest.
And this evening they appealed to delegates to use the opportunity of a change of Labour leader to argue the case for a new trade union freedom bill, which would include restoring the right to take secondary action.
An early day motion supporting the bill has been signed by 182 MPs so far, and this evening representatives of the NUT and T&G unions joined Mr Crow, Mr Benn and Labour leadership challenger John McDonnell MP to argue its case.
"It's nothing short of shameful that this far into a Labour government we are still working for trade union rights. Eighty per cent of the Conservative anti-trade union legislation is still in place," said Christine Blower of the NUT.
"The UK has been consistently violating International Labour Organisation (ILO) standards, and the submission we are putting in [to ministers explaining this] now are broadly similar to the submissions we put in when the Conservatives were in power," added Sarah Veal, the TUC's head of equality and employment rights.
The trade union freedom bill, which was launched last year, includes six "mild, modest, moderate proposals", in the words of John Hendry QC, who has been helping to draw up the legislation to present to parliament in the near future.
It covers the protection afforded to workers taking lawful industrial action, the use of agency workers to cover striking employees, the protection of unions from court actions because of a strike, reviews the rules on ballots and defines a lawful dispute.
Finally, it would allow workers to strike in support of other workers in their union who have different employers. This so-called secondary or solidarity action is currently illegal, but unionists argue this ban is illegal under international law.
"Secondary action is a fundamental part of trade unionism, but it's more than that - it's a fundamental aspect of human behaviour," Mr Hendry said.
When the bill was first mooted, last year, then trade and industry secretary Alan Johnson rejected its proposals outright, insisting there would be no return to the industrial discontent of the 1970s.
But this evening Mr McDonnell - who is challenging Gordon Brown for the leadership on a traditional Labour ticket - insisted that despite having no support from members of the cabinet, the bill was popular among trade unions.
"When we publish the bill we will work with MPs, and liberate Labour MPs to vote with their conscience for the first time in nine years," the Hayes and Harlington MP said.
He added: "The trade union groups have relationships with MPs - they have been there when needed to mobilise voters or funds, but we should now say this is when we need them. On that basis I believe we can carry [the bill]."