A militant railway union boss vowed the 'fiercest resistance possible' to Government plans for a law change designed to prevent strikes from crippling the country.
RMT kingpin Mick Lynch lashed out after Grant Shapps suggested agency workers could be used to cross pickets and keep services running in future.
The Government is considering legal changes, with travellers facing chaos on the railways and London Underground due to widespread industrial action.
The move - which would not be ready in time for planned June shutdowns - would involve reversing a restriction preventing employers from hiring agency workers to cover for striking staff, and would apply to all sectors, according to The Sunday Telegraph.
Such a move would ramp up the Government's growing battle with public sector unions demanding huge pay rises at a time when millions of families are facing hardship.
RMT general secretary Mr Lynch accused the Transport Secretary of 'smearing' its members, adding: 'We already have the most restrictive anti-democratic trade union laws in Western Europe and if the government attempts to reduce our rights further, the RMT along with the rest of the trade union movement will mount the fiercest resistance possible.'
It came as Labour remained bitterly divided over whether to support or oppose the action by a union not affiliated to it.
Shadow chancellor Rachel Reeves attacked the agency worker plan, saying passengers would not feel safe on services driven by agency staff.
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