In principle, the workers of the contractor have no lien on employment with the principal employer, i.e., the company. Their contract of employment is with their employer, i.e., the contractor. Therefore, as soon as the contract with a company is terminated, the contractor will have to address the issue of either deploying the workers (of the contractor) in other plants or retrenching them. In this process, the principal employer has no obligation. The workers of the contractor also do not have any right to raise an industrial dispute against the principal employer, though they can raise it against their employer, i.e., the contractor, for non-employment, etc.
Practical Implications of Union Disputes
However, in practice, in such scenarios, the union will raise a dispute against the principal employer only, knowing that it is just illegal. But a strike, being a manifestation of the right of workmen, will get support from the media and the public. They are not concerned with what is written in the law, but for them, those who work for the company are workers of the company only. If it is established that the same workers of the contractor were being controlled and supervised by the supervisors of the company, the contract would become a sham contract, leading to making the strike a legal one.