Unions organising strike action in monopoly public services such as those provided by councils could be sued for damages by the public and by employers, under Mr Lang's proposals.
It is not clear whether the proposals will apply only to national strikes, or whether they will affect contracted- out services and quangos.
'It's confused, ill-thought-out, and a fundamental attack on human liberties,' said Unison head of local government Keith Sonnet.
GMB general secretary John Edmonds said: 'We're not sitting here working out what it means because it won't happen. This whole thing is a sideshow to detract from the Tories' mess over Europe.'
GMB national secretary Mick Graham said: 'The statement that [Mr Lang] made is so very vague that it's obviously a pre-election gimmick.' He suggested that if unions could be sued for strike damages, employers should be too.
Mr Lang said he would publish a package of proposals after Parliament resumes, hinting that they could be included in the Queen's speech on October 23. He said he will begin to consult immediately on the new measures, which will embody what he described as 'a new concept in British industrial relations - that of proportionality'.
'If a strike's effects are disproportionately damaging to the public, then the trade union which organises the strike will risk losing its immunity and being sued for damages,' Mr Lang said.
'I am determined to ensure that if, in future, trade unions choose recklessly to overstep the limits of legitimate protest by exploiting monopoly power, they will face the consequences of their actions.'
He said Tory anti-union legislation had not done enough to break the power of unions in the public sector.