Unions have reacted furiously to new legislation that would tighten rules on strike ballots and political donations.
Under the Trade Union Bill, a turnout of at least 50% of members will be needed to authorise action.
Yvette Cooper, one of Labour's leadership candidates, said that the government's plans for trade unions are an "ideologically driven attack on Britain's trade unions" which "puts narrow Tory party interests ahead of what is right for the country".
"It not only undermines years of progress on workers' rights, but it also breaks the growing consensus on the need to reform funding of political parties.
"This is another example of the Tories employing cheap divide-and-rule tactics."
The motive for the changes to strike rules may be more complex than the government is prepared to admit.
The Trade Union Bill is getting its first reading in Parliament latter today. Various strict new measures are expected.
The Government has come under fire as it launches its biggest campaign against trade unions in 30 years.